I think riding my heavy Trek around for the past couple of weeks has really helped build up my core. Today I took my Scott out for a spin and ended up riding around lakes Lemon and Griffy. This is a very hilly route and I really didn't experience any fatigue in my legs like I normally would when I don't ride my Trek. Riding the road bike is more about speed, while riding the Trek is more about taking one's time and enjoying the ride. In terms of fitness, the trade off by riding my lighter road bike is that I'm not really building any muscle up unless I'm climbing hills, so I'm mostly using my upper dexterity muscles. One the Trek, I have to constantly push to keep it moving and I'm using more of my core muscles.