How do you keep from coasting too much on a bicycle? Something that has always confused me about working out on a bike.  I want a Hybrid to ride for fitness.  I do not want the Road bike, mostly because I'm not going to ride on the road (I am too afraid of being run over).  The Hybrid will be on paved trails.  Plus, my back would not survive leaned over on a Road Bike, though I wish I could.

My question is how do you keep from coasting too much?  Whenever I think of biking, I inevitably think of hitting hills eventually.  Granted I have to go up the hill, but then I'm coasting down.  And on flat, how do I keep the wheels moving without coasting?  Just the right gear and discipline?

One can only go so fast, so sooner or later I will have to stop pedaling, and I also don't think I can go full out all the time.
You are correct - keeping the bike in the appropriate gear is important. Generally, you should have a preferred pedaling rpm, or cadence that you try to keep constant. Change to lower gears on uphill gradients, higher gears on downhill gradients. 

If you find yourself going downhill at a speed that even your highest gear makes you pedal too fast, feel free to coast. There is no rule that says you have to spin your legs madly to keep pedaling. Hybrid bikes tend to have fairly low gearing so coasting on faster downhill sections is normal.