Then WHY buy a mountain bike??!! Go to your local bike shop & ask about a hybrid or performance hybrid. Some now call these "fitness bikes". MUCH faster on the streets. NO suspension parts to zap your energy. Plus the taller & thinner 700C tires will roll much easier with less resistance than knobby mountain bike tires.
Don't buy what you want or you think looks cool. Buy what you NEED!
http://www.rei.com/learn/expert-advice/bicycle.html