Honestly, I would either use your current mountain bike (or get a hold of a road bike) in conjunction with a indoor "fluid" trainer. A fluid trainer is different than a magnetic wheel type trainer.
Hook your bike's rear wheel into the trainer then you're ready to start to ride indoors. Use music, a Hindustan chant or something, anything to fight off the boredom than can come when riding no where, and sweating profusely. Also, have a towel handy to wipe your face off periodically, and by all means use an energy drink of choice -- by all means never become dehydrated, because feeling thirsty is the first stage of dehydration. Hence, drink little and often, even if you do not feel thirsty. Try going to www.excelsports.com
and check out their trainers. Hope this helps.