I started a thread a couple of years ago on this subject. The thread, which went on for 11 pages was titled " a question regarding "continuance"".
There were many helpful suggestions made --
I think my personal greatest hindrance to swimming lap after lap is a level of dedication less that what it should be. With greater dedication in combination with all that was suggested, I feel my continuance would be .... continuous.
I have improved quite a bit and 50 M is matter of fact still but much more is a struggle. Now that I am back at the "summertime" pool I will be working on this each day.
All the best ...
If you're not swimming; then you should be skiing......