Personally, I feel like I get as good a work out in just 3mi on a technical mountain trail as I would doing 15 mi on the street. It depends on if you want aerobic or anaerobic excersize and what kind of time you have.

I work full time, am a grad student and I have three sons. Time is not my most abundant luxury. As long as you are consistantly doing something you are going to get in better shape. Just make it fun and never give up.