What is the toughest distance to race?

The steeplechase was by the toughest event I ever ran.

Any distance, you just have to run it hard enough.

500m or 600m IMO.

I've never raced a 500 or a 2,000 but people who have have told me they're brutal. A 500 is basically a 400 where you just hang on for another 100 and a 2,000 is basically a mile with an extra lap tacked on. I can't comment from experience but if true, yeah, those are tough. I do know that the interval session I hated most was 660s.

define "toughest". Most painful is the 800. Most brutal (destructive) is the marathon.

This is correct but assuming you don't run any hard enough the steeplechase might be the answer.

There are different types of tough. Ive done 126k where my muscles were spasming everytime i tried to run after 115k. And anybody who has hit the wall in a marathon will tell you finishing that race, even if they had to walk it, was a true test of resolve.

Then there is a 5k where if you push yourself hard enough your heartrate hits maximum and stays there for the final 1km.

I dont race any less than 5k but ive heard people make a case for 800m too.

Outlier race: Quadruple solo Badwater or the "hundred" miles of Barkley. The last 10 "yards" of a competitive backyard ultra.

Common race: 800m

Or really racing a marathon on a hot day, but that -- like Badwater or Barkley -- is as much about conditions as distance.