I'm not sure I understand whether the OP's average speed has gone up or down with the increased leg speed, but it doesn't matter, and is nothing to worry about at this point.
My advice to the OP is to keep at, and to push yourself in one way or another. Interval training, where you go hard for a bit, then easy, then hard again, is a great training tool, as it conditions your body both to stress and to recovery. Do that on some rides, then on others, spend a ride working on high cadence, another on going easy and longer distance; mix up what you're doing on each ride, so that you train different abilities and don't burn out by pushing continuosly for some random target which is really insignificant to your development as a rider, or which may even be unattainable given your current situation (i.e. conditioning, equipment, routes, etc.).
When doing intervals, consider pushing hard at lower cadence (65-75rpm) for a couple of minutes, right up until that "heart attack" feeling, then shifting gears, easing back on the effort with a higher leg speed (80s rpm) and let your heart rate come back down and catch breath for 5 minutes, then do it again. Repeat that alternating scenario for several times, like 4 each, and you'll pass a good 35 minute ride hopefully feeling like you made a good effort, but didn't wipe yourself out. That'll do a lot of fitness building for you.
As you become more comfortable with the efforts, alternately turn up duration and intensity of the hard intervals, but keep the easy recovery part easy, that's important. My guess is that by the time you're ready to do that, you'll be looking at riding differently, and eyeing hills and little rises as opportunities to drop the hammer! You'll start enjoying those hard efforts, confident in the knowledge you can go hard and still feel good and recover after.