I think if you eventual goal is fitness, there's no point in making your FS MTB more efficient. Once you're in a condition to ride that fast, you'll undoubtedly ride a road bike even faster (once you get used to the different posture on a road bike). For you price given, I think it's best that you look for a used road bike. If you want to build up your legs for cycling, then ride more. Of course cross training never hurts.