Yes you need to be fit to ride dirt bikes. Toughest sport ever? I'll argue that. I can not ride a month and race a moto. Can't do the same with a 50 mile Cat 3 critirium.
Check Dilla in 1977
The racers had to ride the banners to miss bumps.
10 inches of bad travel and drum brakes. I can't remember the last time I saw a modern Pro track that rough.
I think they dumbed mx down to keep the pack closer together. Good guys would be further off the front on a tougher track. Doesn't matter anyway. There are less fans at Saturday races and TV can be altered to make up the 3 minute lead