Hmmm. Not much time, but then again, you aren't starting from scratch. I'd say your best bet is to spend some time on the bike at least 6 days a week, which probably means a trainer this time of year. For the first month, don't go hard, just be consistant, no more than an hour a day (apart from the class and group ride). Use the spin class and weekly group ride for the work-out aspect. Second month, incorporate some harder efforts, to build strength and anaerobic capacity, but no more than 3 days a week (including spin class and group ride). Keep riding at least 6 days a week, but go easy between work-out days so you recover. You will make faster progress undertraining than overtraining. Good luck, and regardless of how successful your preparation, enjoy Moab. He will probably want to go out and hammer some of the slickrock. Decide now to be cool with that, be sure he knows it, and you'll both have a better time.