No advice, the longest I've been on the bike since my whiplash injury in October was 90 minutes and that was on the road. I do, however, want to wish you good luck and have fun while training!

I found that, for me, when riding longer than two hours was to have liquid nutrition in the form of Accelerade and something like a broken up Cliff bar or two and some shot blocs ready for me to consume. That was for very long road rides though, mountain biking uses more energy...