jeez. this does not sound friendly at all. it doesn't even sound like fun. but...

how rough are the trails and what sort of bike to you plan to ride? depending on the offroad section, if you ride a rigid 29er with 'cross tires on it, you will probably have a big advantage over him if he rides a 26 inch mtb.

if he actually rides 120 miles / week, that is pretty good, but that doesn't necessarily make him fit. if you actually train for tri's and go out and hammer on the road, you are going to spank him. seriously. if i were you i would just make sure you are prepared to deal with mechanical problems and outsmart him. it will probably be really easy.

as for rules...hmm. each rider must be completely self-sufficient? that is...no aid, no van backup, no pits, no staff of mechanics, etc.

what about interferences like local traffic, peds, etc?