Motocross racers must be extremely fit. The sport is very demanding on the arms, legs, shoulders, and glutes. The rider has to be able to keep control of his machine on a course that most people would have difficulty walking across.
If racing cross-country through a mix of both open and tight Mallee Bush tracks appeals to you, then you should consider Enduro riding. It’s a case of rider and bike verses the elements of the natural terrain.