I just got a MTB and I hope I never "have" to do a century on it ;-)
I commute on the EAI fixed gear in my signature, tour and train on a touring bike and will hopefully be having a blast in the forest/winter on the mtb.
I started out at just below 300 and have lost a lot.
My approach was to have fun first and enjoy the fitness/weight loss benefits as a bonus and it's working so far, so just go out and have fun :-)
Anyways, welcome to the group and congratulations on an investment in a healthier lifestyle.