I started riding my 20 year old ten speed (laugh, but it's comfortable and fun) last year again, mostly for the fitness and pleasure of being outside.

I'm trying to figure out what a good fitness level speed/distance to aim for would be; I live in the midwest, so not much in the way of hills, and ride mostly on bike trails. So far, a good ride for me is 15-20 miles at about an average of 15mph (the trail's marked, and some friends got me a bike computer last summer to add to the fun); that's me working hard, but still enjoying the ride.

When the weather's good, I try to ride at least every other day, but it's the midwest, so...

One of my acquaintances talks about peddling at different speeds. Can someone explain that a bit, please, and should I worry about that for basic fitness and fun riding?

I've been lurking and learning from you all already. Thanks in advance for your help.