PB said:25 metre lap per 45-60 seconds over an hour's time or so...
Is it technique or fitness and lung capacity that improves time ?
I just realized you said "lap." I assume you don't mean 50 meters. You
mean you are able to maintain a continuous pace of 45-60 seconds per
25 meters for an hour.
If so, then, assuming there is nothing terribly wrong with your
stroke, you can improve your speed by doing some of the things
suggested in this thread. Namely, don't just swim laps for an hour.
Rather, maybe swim laps for a half hour, and then do some sprints.
You could go 10x25m, resting 30 seconds after each 25m. Swim each one
as fast as you can go.