^^This - except it isn't speed that matters, but intensity. As others have said, there are too mny variables to make speed a useful indicator unless one is covering the same course under similar conditions. Hills and headwinds are the big factors, of course: a big rider is at very little disadvantage on the flat, but hauling that weight up a 10% gradient takes plenty of doing.

If you want to get faster you have to increase your threshold power output, which means training intensely - intervals, hills, whatever - a couple of times a week. But you shouldn't try to do that all the time, you need to recover. So the best idea is to spend most of your week doing LSD - long steady (not slow) distance, the longer the better, if you have the time - and throwing in a shorter, much more intense session on one or two days. That's probably optimum for building an aerobic base, losing weight and increasing power.