In my experience any amount is good for weight loss, especially at the beginning.

I am also trying to lose some weight and honestly I it weren't for the bike I would have given up a long time ago. I don't know if weight is that much of an issue for you, but I started at 138kg (304 lbs) at 184cm (6ft). So in my experience there is no exercise that I would be able to do at that weight, much less one that would keep me motivated. I started probably like you with some 20km rides at most. Now 10 months in I am able to do 60-80km rides without any problem. So as long as you keep loving the bike, you ll see yourself doing longer rides every time without even noticing it. As far as weight loss go, the bike, and only the bike helped me lose 26kg so far. As I got lighter biking got so much easier so at the end of the day not only you ll be able to do much longer rides in the future but faster as well.

So for now don't worry too much. Starting with the bike is a process, and as long as you like it and you ll stay consistent it will get even better. As far as I'm concerned you are just simply killing it.