An updated plot of push-up statistics:
I had some problems with my wrists last week, so I had to take a bit of a break. But this has resolved itself and slow progress is being made again. Doing 25 push-ups in a row is already more than I was ever able to, I think.
The thing with the wrists is kinda funny. Whenever I start a new exercise, something unexpected, other than the training goal itself, becomes the limiting factor for a while. This was very clear when I started running; at some point it was the shins, then the calves, or the knee, once even the bones in the foot's arch. My way of handling it is always the same: take it easy, don't push through it. Try to improve form, that is try to figure out if you're doing it wrong. After a while the body adapts and the next thing becomes the limit. Repeat.