If you don't already, you might consider getting a bike computer with a cadence moniter. No need to be a slave to it, but I found that it greatly improved my shifting -- when you see your cadence drop (or rise) significantly it's a good cue to shift, even if you feel like you **could** just muscle through it. You'll be able to go farther and not get as tired, and it will put less stress on your knees. As far as ranges go, I think you have to determine for yourself what is comfortable and I imagine there is individual variation. For me, my cadence is almost always around 90 on the flats, I try not to drop too much below 70 on hills unless they are real killers, and at anything much over 100 I feel like I'm "pedaling air" as you say, and not getting enough power from each stroke.

As far as muscle development goes -- my legs were already fairly lean and muscular from running (and years of ballet and cross-country ski racing when I was younger), but by the end of the summer last year (my first season cycling), I could see parts of my quads I never knew existed. I think it's a little like running; in the same way that sprinting builds power and running long distances builds muscular endurance, mashing will tend to develop short-twitch muscles while spinning probably does a better job training long-twitch muscles. If you want both try spinning up hills .

Good luckl