Dennis Yu

How I Finished an Ironman 70.3 on Zero Training (and a Facebook Marketplace Bike)

I signed up for an Ironman 70.3 in Idaho with my friend Marko, and I did it on zero training. Not “a little rusty” training. Zero. No running, no swimming, no biking. I didn’t even own a bike when we got on the plane — I bought one off Facebook Marketplace the night we landed in Coeur d’Alene, so I’d have something to ride the next morning.

Seventy-point-three miles: a 1.2-mile swim in Lake Coeur d’Alene, a 56-mile bike, and a 13.1-mile run to finish. I was about 70 pounds overweight. And I crossed the line.

People hear that story and assume it’s a story about being reckless. It’s the opposite. It’s a story about determination and smart risk assessment — the same two things that decide almost everything I build today.

70.3
miles raced
0
hours trained
7:51:05
finish time
204
race bib

The bike came off Facebook Marketplace

This is the part people can’t get past. The night before an Ironman, most athletes are laying out a bike they’ve trained on for months, checking tire pressure, obsessing over their nutrition plan. I was scrolling Facebook Marketplace in a hotel room, messaging a stranger about a used road bike, then driving over to hand him cash. The next morning I rode that bike 56 miles around Lake Coeur d’Alene.

Dennis Yu riding the bike he bought off Facebook Marketplace along Lake Coeur d'Alene during Ironman 70.3
The bike I bought off Facebook Marketplace the night we landed — on the 56-mile course around Lake Coeur d’Alene.

Was that ideal? No. But it wasn’t reckless either, because I had already done the risk math. A bike just has to roll and brake. I wasn’t racing it — I was going to spin it gently for a few hours. The downside of a cheap used bike was low. The upside — being able to start at all — was the whole thing.

The athlete I used to be

Here’s the context that made the risk acceptable. Twenty years ago, I ran Division I cross country and track at SMU. We sometimes ran over 100 miles a week — two runs a day, starting at 5 a.m., plus weight training. I ran the 3,000-meter steeplechase, hurdling barriers and leaping the water pit lap after lap.

Dennis Yu running on the track for SMU as a Division I cross country and track athlete
Two decades earlier: running for SMU. Your body remembers how to suffer, even when it’s buried under 70 extra pounds.

That body was a long way back in the rearview mirror. But endurance leaves something behind. I knew what deep fatigue felt like and that it doesn’t kill you. I knew how to pace, how to breathe, how to keep my feet moving when everything wanted to stop. You don’t lose that knowledge when you gain weight. You just have to be honest about what you can and can’t ask of the body you’re standing in today.

This was risk assessment, not recklessness

The reason I could start an Ironman 70.3 with no training is that I wasn’t trying to win. I was trying to finish. Once you separate those two goals, the whole risk profile changes.

So I managed the things that actually hurt you:

  • I never went into the red. The real danger in an endurance event isn’t being slow — it’s going too hard when you’re unprepared. I kept every effort easy and aerobic, all day.
  • I drank constantly. I had plenty of water and electrolytes the entire way. Dehydration is what ends most under-trained athletes, so I never let myself get behind on it.
  • I stretched and I rested. I took the time to loosen up when I needed it, and I’d slept well going in. I treated the day like a long, deliberate effort, not a sprint.
  • I respected the clock, not the crowd. I knew the cutoffs, and I knew that walking the run was completely fine. Slow finishers still finish.

I wasn’t trying to win, so I took it easy the whole way — just to finish. That single decision turned an “impossible” race into a manageable risk.

The day itself

I swam. I rode the Marketplace bike. And then I ran — 13.1 miles, 70 pounds heavier than the guy in that old SMU photo, in front of strangers, with no business being out there on paper.

Dennis Yu running the Ironman 70.3 half marathon roughly 70 pounds overweight
Dennis Yu on the run course at Ironman 70.3 Coeur d'Alene

No hiding it: this is what 70 pounds overweight and under-trained looks like on the run course. I kept moving anyway.

I won’t pretend it felt good. It didn’t. But “uncomfortable” and “dangerous” are not the same thing, and I had built my whole day around staying on the right side of that line. Every mile I wasn’t trying to be fast. I was just refusing to stop.

The finish line

Seven hours, fifty-one minutes, five seconds after I started, I ran under the Ironman 70.3 arch in Coeur d’Alene. Not fast. Not pretty. But a finish is a finish, and the clock doesn’t ask how you trained.

Dennis Yu crossing the Ironman 70.3 Coeur d'Alene 2024 finish line
The Ironman 70.3 Coeur d’Alene finish — 70.3 miles, zero training, one very determined finish.

What it actually taught me

I tell young people and business owners the same thing all the time: you can get almost anything done with determination and smart risk assessment. This race is the clearest proof of it I have.

Most people overestimate the preparation they need to start, and they underestimate what steady, low-risk persistence can finish. They wait until they’re “ready.” They want the perfect bike, the full training block, the guarantee. So they never get to the starting line at all.

Smart risk assessment flips that. You ask: What’s the real downside here? Can I cap it? Can I remove my ego and just aim to finish? When the answer is yes — buy the bike, hydrate, take it easy, and go. It’s the same move whether the finish line is a race in Idaho or a 19-year-old launching their first business with tools they’re still learning to use.

Assess the risk. Remove the ego. Keep moving. Finish.

Where I am now

That finish line restarted something. Over the last several months I’ve taken most of the weight back off — not for a race, just for me. The medal was never the point. The point was proving the operating principle I live and work by: you don’t need to be the most prepared person on the course. You need to be honest about the risk, refuse to quit, and keep putting one foot in front of the other until you’re across the line.

Here’s what that looks like now — same operating principle, a healthier starting line:

Dennis Yu in 2026, slimmer, with three young AI Builders at Grill Nine
These days — most of the 70 pounds gone, usually surrounded by the young builders I work with.
Dennis Yu in 2026 at a student event, having taken off most of the weight
Still showing up, still chasing finish lines — just a lighter version of the guy out on that race course.

If you’re staring at something that feels impossible — a race, a business, a comeback — don’t ask whether you’re ready. Ask what the risk actually is, how to cap it, and how to start. Then go finish.

Scroll to Top