Originally Posted by
kensuf
To add to this.. You said your friends wanted to be competitive in this event. That means the pressure will be on you to perform as best as you can, which opens up those risks.
If you were just doing this for chuckles and grins then I'd say go for it, but since you plan to actually race it...
that is a valid concern, but notice that the op asks us to only consider the fatigue aspect. i don't think that a six hour mountain bike ride two weeks prior to 'a' race day is going to adversely affect his fitness. i also don't believe that a 1x a week mountain bike ride in place of a shorter training ride will hamper his performance.