I worried some about doing 200K ... but there's some fundamental difference on a bike, maybe that ability to coast if you want, or not being weight bearing. I think that every bit as important as fitness would be those "comfort" issues... you're doing a LOT of repetitive motion. On the right bike, it's almost all fun at 200K - boredom is another thing to plan for, but I suffer less from that (any terrain is more interesting than laps in a swimming pool!)