Is 300km with 12,000' of elevation long and gruelling enough? How about 400km?

My commuter bike is my long distance bike; a Surly Cross Check outfitted with a randonneur rack and Berthoud 12L front bag, and a Carradice Pendle (11L) bag on a Bagman support rack in the rear. I have 32mm tires, full fenders and mudflaps, and generator lighting. I routinely ride 200km, and have gone as far as 400km in a single day.

One of my century partners (up until recent injuries) rode all his distance mileage on a Specialized Stumpjumper with a lockout suspension fork and Mavic SpeedCity wheels with 25mm tires. As long as you don't have any expectations of a fast finish, even a MTB or a hybrid that you're comfortably fitted on is fine for distance riding.