How I built a self-adapting training plan in Next.js (no AI, just logic)

Run_Dev — Thu, 30 Apr 2026 07:13:48 +0000

How I built a self-adapting training plan in Next.js (no AI, just logic)

When I started building RunAdapt, I had one goal: a running plan that doesn't fall apart when you miss sessions.

No machine learning. No LLMs. Just deterministic logic that runs every time you mark a session as MISSED or PARTIAL.

Here's exactly how it works.

The core idea

Most training apps are static. You get a plan, and if life gets in the way, you're on your own. RunAdapt runs 4 rules in sequence every time you miss a session.

Rule 1 — Reschedule

Finds the next available slot in your calendar and creates a new session there.

if (session.status === 'MISSED') {
  const nextSlot = findNextAvailableSlot(plan.availableDays);
  createSession({ ...session, date: nextSlot });
}

Rule 2 — Reduce intensity

If you have 2 or more consecutive missed sessions, the intensity factor drops by 0.15 (minimum 0.60). No point pushing hard if you're already struggling.

if (consecutiveMisses >= 2) {
  plan.intensityFactor = Math.max(0.60, plan.intensityFactor - 0.15);
}

Rule 3 — Adjust target date

If your failure rate exceeds 20%, the race date moves forward up to 8 weeks. Better to adjust the goal than to pretend you're on track.

if (failureRate > 0.20) {
  plan.adjustedTargetDate = addWeeks(plan.targetDate, weeksNeeded);
}

Rule 4 — Micro session

If you have less than 30 minutes available, instead of skipping entirely, the plan creates a 20 min / 3 km session. Something is always better than nothing.

if (availableMinutes < 30) {
  createMicroSession({ duration: 20, distance: 3 });
}

Why deterministic logic instead of AI?

I considered using an LLM to make the adaptation smarter. But for this use case, deterministic rules have real advantages:

Predictable — the user knows exactly what will happen
Fast — no API calls, no latency
Cheap — no token costs
Trustworthy — no hallucinations

Sometimes simple is better.

Stack

Next.js 14 (App Router)
Supabase (auth + database)
Vercel Hobby (free)

What's next

I'm currently looking for my first real users to validate whether these 4 rules are actually enough — or if I'm missing something obvious.

If you're a runner, I'd love your feedback: runadapt.vercel.app

I built a running app in 13 days because I kept quitting my training plans

Run_Dev — Wed, 29 Apr 2026 14:49:27 +0000

I built a running app in 13 days because I kept quitting my training plans

I've been running on and off for years. My biggest problem was never motivation — it was that I'd miss two sessions and the whole plan would fall apart.

Every. Single. Time.

I'd look at the plan, see I was already 3 sessions behind, and just... stop. Sound familiar?

So I built RunAdapt

RunAdapt is a training plan that adapts automatically when you miss sessions:

Miss a session? It reschedules it.
Miss two in a row? It lowers the intensity.
Too far behind? It moves your race date instead of pretending you're still on track.
Less than 30 min available? It creates a shorter session so you don't skip entirely.

No AI, no black magic. Just smart logic that keeps your plan relevant no matter what life throws at you.

The stack

Next.js
Vercel (Hobby plan — free)
Supabase

Built in 13 days from scratch.

What I learned

The hardest part wasn't the code — it was figuring out who the app is actually for. After two days of Reddit feedback I landed on this: people who have been starting and quitting running plans for years, not because they're lazy, but because the plan never forgives you for being human.

Still looking for my first real users and brutal feedback.

runadapt.vercel.app

showdev #beginners #webdev #running

DEV Community: Run_Dev

How I built a self-adapting training plan in Next.js (no AI, just logic)

How I built a self-adapting training plan in Next.js (no AI, just logic)

The core idea

Rule 1 — Reschedule

Rule 2 — Reduce intensity

Rule 3 — Adjust target date

Rule 4 — Micro session

Why deterministic logic instead of AI?

Stack

What's next

I built a running app in 13 days because I kept quitting my training plans

I built a running app in 13 days because I kept quitting my training plans

So I built RunAdapt

The stack

What I learned

showdev #beginners #webdev #running