I'll give the obvious advice, also, that I don't think anyone else has. If you've been pretty sedantary for a while and are pretty much deconditioned, it might not hurt to see your doctor just to make sure he's OK with the amount of exercise you are considering. Understand that no doctor in the world is going to tell you not to exercise, but you might run your plans by him and see what he says.

Beyond that, you can absolutely make it. There is no substitute for time on the bike. The MS rides aren't a race. You just want to ensure that you can make 75 miles in a day and then go do it again the next day. The best way to do that is build up to it slowly in training. Ride every night, at least a little, if possible until you are able to do weekend rides of at least 75 miles. You can absolutely do it.