Everyone starts together, finish as fast or slow as you'd like, as long as it's within 90 minutes. And at the end of that 90 minutes, you get to start again. Doesn't sound too tough, right? Right. But what happens when you don't make it back within that 90 minutes? You're out. No more running for you. What happens if you make it back early? Sit down, eat some grub, take a nap, flirt with the locals, do what you need to do. Just be ready to go again when those 90 minutes are up.