it took me 30+ years to realize that I'm a sprinter, not the type of person that can really expect to go 40km for an extended period by myself. More like 65km/hr for a couple hundred meters. At my peak, I could barely manage solo 40km/hr for an hour, and that was after a tremendous amount of training. Now, I find myself just enjoying riding my bike, although I would like to be faster.
Which is to say, try to be patient, lose some weight, build up slowly. You have many years to reach a goal. I would expect to reach some level of stagnation if you try to take it too fast. In fact, it's really hard not to get into a period of stagnation no matter what you do.