30km/hr is pretty good after only 6 month of riding. it's approx. 20MPH.

One thing I've noticed over the years is that our improvement is not necessarily linear. it tends to be more like staircases. See some quick improvement. Get stuck at that level for a while. Then suddenly show some improvement again. While you are in the plateau, your body is settling into and getting used to the new fitness level.

Just keep riding and once a month or so go for extra hard or longer distance. This will help you get past the plateau.

Again you are doing pretty well.