Well since you have just gotten around the joys of toe clips and are still riding short distances, you sound like a relative newby to me.

OK you are not totally green. But learning to ride does take time. And developing a good cadence is important. On fast rides, I just concentrate on either keeping up (if I am a lagger) or killing them (if they are lagging me). I don't work much on form.

However, you should do some relatively easy rides, and you can work on getting your spin up on those. Just ride in the easiest gear you can and spin as fast as you can (without killing yourself). It will come. A computer with a cadence option helps here.

Generally riding faster has to do with your cadence, your overall fitness - you can improve this by riding most days or doing some aerobic exercise virtually everyday, and your strength. You can increase strength by going out and riding fast rides with the local nuts. Nothing like trying not to get dropped to give you a good workout. Don't do this all the time. It hurts. If you hurt yourself everytime you ride, you will figure it out and stop riding.

So work on your cadence - often in recovery or easy rides, ride everyday it does not have to be hard (ok you can take a day off now and then) and ride fast - bout once or twice per week - either do intervals or chase a pack with yer tongue hanging out. It is simple and it works. Oh yeah - you can throw in a weekly long ride too. After awhile you will come up with a routine that best fits your needs.