The 42K
Marathon strategy sim · UE 5.8 · 2026
The marathon from Gold Medal Challenge ’92, rebuilt as a strategy game. You don’t mash buttons; you manage a body over 42.195 km, while an AI announcer calls the race live.
Role: solo designer: GDD, every design decision, tuning, playtests, art direction. Built with: UE 5.8 Blueprint, a Python reference model, Claude Code; live commentary by Claude Haiku. Timeline: playable race in 3 days (Sep 27–29, 2026), polish through Oct 17. Status: packaged Windows build; Career and Classic modes playable.
| By the numbers
4 decimals
The Blueprint race matches a Python reference model to 4 decimal places. Same seed, same race.
96%
Live announcer lines accepted by the guardrails · 0 repeats · 1.06 s median
280
Headless AI-vs-AI races (40 at each of 7 difficulty values) set the shipped difficulty curve
3 days
From a blank project to a playable race
| The design house
1. Readable, not optimal
The AI makes visible moves (breakaways, drafting, fading) that the standings and the announcer can explain.
2. Same rules for everyone
The AI runners use the exact same simulation as the player.
3. Every choice is a wager
Shifting pace zones costs stamina, so the zone you commit to is the bet.
| Inside the design
The fun
The 1992 original was a button-masher. The interesting part of a marathon is the decisions: how hard to push, whose slipstream to sit in, and whether to lose time at a water station now so you don’t hit the wall at 28 km. So I cut the mashing: a pace dial, a rival to draft, aid stations, and a deterministic simulation that turns those choices into a race.
PLAYTEST (3 PLAYERS): THE WATER AND GEL DECISIONS LANDED
Decision: the Wall is a delayed consequence
Your hydration at 28 km sets a stamina cap for the rest of the race: about 93 if you drank at every station, 35 if you never did. Skipping water is fine… until it isn’t. And heart rate multiplies drain above 150 BPM, which is what makes water and pacing worth thinking about.
D-16 · D-12
Decision: drafting is a contract
Locking onto a rival is free and cuts your drain by 25%, but your speed follows theirs: if they surge, you surge and pay for it. A seeded test found what no human would catch early: in 7 of 20 races, runners drafting each other in a loop slowed to a crawl. The fix was a chain check, and the player now sees “DRAFTING YOU.”
D-06 · D-08
Proof: a live AI announcer with guardrails
Claude Haiku writes commentary from the race facts in about a second. Lines must fit 52 characters, use only characters the pixel font can draw, contain no invented numbers and name no runner who isn’t in the race. A late or rejected line falls back to a hand-written one, and the race result is never affected. In a full race, 25 of 28 lines were live.
SYSTEMS · LLM GUARDRAILS · EVAL