Rans VRex. Get a lightly used one off Craigslist.

Anyway, kudos to you. I've been riding road bikes for years, did a century two years ago, have a new recumbent bike, and am slowly working up the mileage. I'm also pretty brave. I won't attempt a century until June or July if all goes well. You signed up for a century 4 months from now armed with an ill-fitting mountain bike. You are very brave.

It can be done, mind you. You have very aggressive goals. Go for it.