I think genetics is a paramount concern when talking about competition or
your own training.  For example, two years ago I ran a marathon, training
for about 6 months.  My friend ran also, and his only training was smoking
cigarettes - he did not run once.  Yet he still ended up beating me! The
physical differences along the genetic spectrum are tremendous.

This is also a major concern for training.  Different people have different
capacities for stress.  When formulating an exercise program, you should not
base it on someone else's program.  Some people might be able to work out
5 days a week, some may only be able to work out 2 days.  Art has described
the principles of EvFit.  Everyone should adapt them to their own
circumstances, and see how they work for them individually.