Besides weight I would also look at gearing. I've done the MC several times and I'm 5'6" 175 - 180. If you can't finish with your easiest gear, then you need more, simple as that. Not everyone can push a 39 - 25 or whatever up decker, use what will work for you and the only way to know that is to experiment with your gearing. The length of climbs on MC can last for a good while 20 - 45mins a pop depending upon your strength. What I would be aiming for in rides is basically about 6k - 7kft of climbing in 60/70 miles or so once a week. During the week try to do hill repeats 3 x 20mins or 2 x 30mins. Add a recovery ride of about 1 - 1.5hrs or so. After 60/70 miles you need to gauge how fresh your legs are relative to your ride. Steady climbing power is what you are looking for.
Ian in SD