i bought a trek 7500fx for my first bike and love it. Used it for the empire st aids ride last august, did fine (560 miles in 7 days) and was generally in the middle of the group each day.

That said, i wish i'd gotten a road bike instead. i never expected to ride more than 30 miles a day. What did i know? Yeah, it's handy to have a bike that is a bit more sturdy (i live in nyc) and less likely to flat, but overall i'd suggest going w the best road bike you can afford.