How I Built a Free AI Fitness Coach with Claude That Brings Over Structured Trainings to My Watch

The Problem: Good Coaching Is Expensive

When I decided to train for a marathon with an ambitious time goal, I faced a dilemma. A professional running coach costs €150-300 per month. Generic training plans from the internet don’t adapt to your life – they don’t care if you slept poorly, got sick, or had to skip a session because of work.

Then I stumbled upon a video by Wild Rapha on YouTube where he showed how to use ChatGPT as an AI running coach. I loved the concept, but I wanted to take it further: I wanted to use Claude instead, and I wanted a coach that doesn’t just give advice, but actually controls my training calendar and sends structured workouts directly to my sports watch.

After a few weeks of experimentation, I now have a system that:

• Plans and adjusts my training sessions based on my current fitness level
• Reacts flexibly to life circumstances – whether it’s poor sleep, illness, or family obligations
• Creates workouts in the correct format that sync directly to my Garmin watch
• Understands my progress and automatically adjusts paces when I get fitter

The best part? It only costs me the Claude subscription I already have.

What You Need (Prerequisites)

Before we dive in, here’s what you’ll need:

Requirement	Cost	Purpose
Claude Pro subscription	$20/month	Access to Projects and longer conversations
Intervals.icu account	Free	Training platform that syncs workouts to your watch
Sports watch	(you probably have one)	Garmin, Apple Watch, Wahoo, etc.
iPhone	(you probably have one)	Calendar acts as bridge between Claude and Intervals.icu

If you already have a Claude Pro subscription, the additional cost is literally zero.

How It Works: The Technical Architecture

Here’s the magic flow that makes this work:

┌─────────────┐     ┌─────────────┐     ┌─────────────┐     ┌─────────────┐
│   Claude    │────▶│   iPhone    │────▶│ Intervals   │────▶│   Garmin/   │
│  (via App)  │     │  Calendar   │     │    .icu     │     │ Apple Watch │
└─────────────┘     └─────────────┘     └─────────────┘     └─────────────┘
                          │
                    Public iCal Link

1. Claude creates structured workouts through the Claude mobile/web app
2. iPhone Calendar stores the workouts in a dedicated “Training Plan” calendar
3. Intervals.icu subscribes to the calendar’s public link and imports the workouts
4. Your watch receives the structured workouts through Intervals.icu’s sync

The key insight: Claude can create calendar events directly through the app. Combined with a calendar that Intervals.icu can read, you get end-to-end automation.

Step-by-Step Setup Guide

Step 1: Create a Dedicated Training Calendar on iPhone

1. Open the Calendar app
2. Tap the Calendars button at the bottom of the screen
3. Tap Add Calendar
4. Name it “Training Plan” (or whatever you prefer)
5. Choose a distinct color so workouts stand out
6. Tap Done

Now make it publicly accessible:

1. In the same Calendars view, tap the info button (i) next to your new “Training Plan” calendar
2. Enable Public Calendar
3. Tap Share Link and copy the public iCal URL (you’ll need this for the next step)

Step 2: Connect Intervals.icu to Your Calendar

1. Go to intervals.icu and create an account (or log in)
2. On the main page, click the dropdown arrow at the top center of the screen
3. Select Subscribe to Calendar
4. Paste your public calendar link from Step 1
5. Save the subscription

Intervals.icu will now automatically import any events from your training calendar. The magic happens because Intervals.icu can parse structured workout syntax from calendar event descriptions.

Step 3: Connect Your Watch to Intervals.icu

In Intervals.icu:

1. Go to Settings > Integrations
2. Connect your platform (Garmin Connect, Apple Health, Wahoo, etc.)
3. Enable workout sync so structured workouts push to your device

Now any workout that appears in Intervals.icu will automatically sync to your watch.

Note on sync timing: The synchronization to Garmin isn’t immediate – it can take some time. If you want to force a sync, go to Settings > Integrations in Intervals.icu, click the pencil icon next to the workout you want to sync, and simply hit OK without changing anything. This will immediately transfer the structured workout to Garmin Connect.

Step 4: Create a Claude Project

This is where the AI magic happens.

1. Go to claude.ai
2. Click Projects in the sidebar
3. Create a new project (e.g., “Marathon Coach 2026”)

A Project gives Claude persistent knowledge across conversations. Instead of explaining your situation every time, Claude just knows.

Step 5: Build Your Knowledge Base

Your AI coach needs four types of files:

File 1: Athlete Profile (update frequently)

This contains everything about YOU:

# Athlete Profile

## Personal Information
- Age: [your age]
- Weight: [your weight] kg
- Height: [your height] cm

## Current Fitness Metrics
| Metric | Value | Last Updated |
|--------|-------|--------------|
| Lactate Threshold Pace | [your LT pace]/km | [date] |
| Max Heart Rate | [your max HR] bpm | - |
| Cycling FTP | [your FTP]W | [date] |

## Personal Records
| Distance | Time | Date |
|----------|------|------|
| 5K | [your time] | [date] |
| 10K | [your time] | [date] |
| Half Marathon | [your time] | [date] |

## Training Availability
- Training days: [X] per week
- Morning window: [time range]
- Limited availability days: [day]

## Goals
- Target race: [race name], [date]
- Goal time: [target time]
- Injury prevention priority: [1-10]

Update this file whenever:

• You set a new PR
• Your fitness metrics change (Garmin auto-detects these)
• Your schedule or availability changes

File 2: System Instructions (rarely update)

This defines HOW Claude should behave as your coach:

# AI Coach System Instructions

You are an expert running and cycling coach helping me prepare
for a marathon with a sub-3:00 goal.

## Your Role
- Understand my complete training plan
- Help adjust sessions when life requires flexibility
- Create properly formatted workouts for Intervals.icu
- Balance optimal training with injury prevention

## Decision Framework for Adjustments

| Reason | Recommended Action |
|--------|-------------------|
| Fatigue/tired legs | Replace with easier session or rest |
| Time constraint | Shorten session but maintain intensity |
| Illness | Full rest until recovered |
| Poor sleep | Reduce intensity or skip |

## Workout Format Requirements

When creating workouts, use Intervals.icu syntax:
- Use percentage-based targets (e.g., 85% instead of absolute watts)
- This ensures workouts scale automatically when fitness improves
- Always include activity type in calendar event title (Run, Ride, Strength)

## Communication Style
- Be supportive but honest
- Offer 2-3 concrete alternatives when adjustments are needed
- Reference specific sessions from my training plan
- Remember: injury prevention is priority 9/10

File 3: Training Plan (reference document)

This is your week-by-week plan. You can:

• Create it yourself in Excel/Google Sheets
• Ask Claude to generate one based on your goals
• Use a proven plan template and customize it

I have mine in an Excel file with:

• Phase overview (Base, Build, Peak, Taper)
• Week-by-week structure
• Day-by-day sessions with paces and durations

File 4: Intervals.icu Syntax Guide

This teaches Claude how to format workouts correctly. The key syntax:

# Intervals.icu Workout Syntax

## Duration formats
- Time: 10m, 30s, 1h30m
- Distance: 1km, 400mtr, 1mi

## Intensity formats
- Power (cycling): 85%, 200w, Z3
- Pace (running): 95% Pace, 4:30/km Pace, Z2 Pace
- Heart rate: 70% HR, Z2 HR

## Structure
Warmup
- 10m 60%

Main Set 5x
- 4m 90%
- 2m 55%

Cooldown
- 10m 55%

Step 6: Upload Everything to Your Project

1. In your Claude Project, click Add content
2. Upload all your files (Markdown, Excel, PDF all work)
3. Claude now has access to your complete coaching system

How I Use It In Real Life

Weekend: Planning the Week Ahead

Every weekend, I sit down with Claude to plan the upcoming week:

“Let’s plan next week. I have a work event on Tuesday evening, so I can only train in the morning. Wednesday is my office day. Create all workouts as calendar events.”

Claude looks at my training plan, considers my schedule constraints, and creates all workouts for the week as calendar events. Within minutes, my entire week is planned and synced to my watch.

During the Week: Adjustments When Life Happens

“I slept terribly last night (maybe 4 hours). Should I still do today’s interval session?”

Claude consults its decision framework and typically suggests alternatives:

Option A: Replace with easy 30-min run, move intervals to Thursday Option B: Shortened interval session (4x1km instead of 6x1km) Option C: Full rest day – one missed session won’t derail your marathon

If I need to swap sessions, Claude updates the calendar events accordingly.

After a PR

“I just ran a 10K in 41:30 – that’s 2:30 faster than my previous PR!”

Claude recalculates predictions, suggests new training paces, and asks if I want updated workouts for the coming weeks.

Getting Workouts on My Watch

The beauty of weekly planning: by Sunday evening, all my workouts for the week are already on my watch. Each morning, I just select the day’s workout and go – no thinking required.

Example: A Real Weekly Planning Session

Here’s what a typical Sunday planning session looks like:

Me: “Let’s plan next week. I need all workouts as calendar events. Thursday I have an early meeting, so I can only do 45 minutes max.”

Claude reviews my training plan for the week and creates multiple calendar events:

Monday - Easy Run:

• Title: “Run - Easy Recovery”
• Description:

Easy Run
- 45m 77-82% Pace

Tuesday - Intervals:

• Title: “Run - VO2max Intervals”
• Description:

Warmup
- 15m 77-82% Pace

Intervals 5x
- 1km 103-108% Pace
- 90s 70-75% Pace

Cooldown
- 10m 77-82% Pace

Thursday - Cycling (shortened due to meeting):

• Title: “Ride - Sweet Spot”
• Description:

Warmup
- 10m 60%

Intervals 3x
- 8m 88-93%
- 4m 55%

Cooldown
- 5m 55%

All events go to my “Training Plan” calendar → Intervals.icu parses the syntax → My Garmin receives structured workouts → My Wahoo Kickr will automatically adjust resistance during cycling workouts.

Total time spent planning: ~10 minutes on Sunday. Manual effort during the week: Zero.

Tips for Best Results

1. Keep Your Athlete Profile Updated

The more current your data, the better the advice. I update after every test or PR.

2. Be Specific With Questions

❌ “Should I run today?” ✅ “I slept 5 hours and have sore calves. The plan shows 6x1km intervals. What are my options?”

3. Use Percentage-Based Targets

Instead of absolute values (250 watts, 4:30/km), use percentages (90%, 95% Pace). When your fitness improves and you update your threshold values, all future workouts automatically scale.

4. Define Clear Rules in System Instructions

The more precise your rules, the more consistent the recommendations. My favorite:

“Injury prevention always takes priority. When in doubt, do less rather than risk injury.”

5. Create a Feedback Loop

After training blocks, tell Claude how things went. It can adjust the plan based on your responses.

Limitations to Be Aware Of

Let’s be honest – an AI coach is not a replacement for:

• Medical advice: Always see a doctor for injuries
• Biomechanical analysis: Running form needs real eyes
• Years of coaching experience: Human intuition can’t be fully replicated
• Accountability: An AI won’t text you if you skip sessions

But for most recreational athletes who want structured training without spending a fortune, this system is a genuine alternative.

Cost Comparison

Option	Monthly Cost
Professional running coach	€150-300
Online coaching platform	€30-50
Premium training app	€10-20
This AI Coach setup	$20 (Claude Pro only)

If you already pay for Claude Pro, the additional cost is zero.

Conclusion

What started as an experiment inspired by Wild Rapha’s video has become an integral part of my training. The combination of:

• Claude Projects for persistent knowledge
• iPhone Calendar as a bridge
• Intervals.icu for workout distribution
• Structured workout syntax

…creates something that feels genuinely like having a personal coach available 24/7.

Will I hit my marathon goal? Time will tell. But I’m confident I’m better prepared than with any generic plan from the internet.

Resources

• Claude.ai – The AI assistant
• Intervals.icu – Free training platform
• Wild Rapha’s original video – The inspiration for this setup (using ChatGPT)
• Intervals.icu Workout Syntax Documentation – Official syntax reference