Endurance swim sets
I can only get a 1hr morning fitness workout a couple of times a week. Can you suggest some sets to increase endurance??
What type of uninterrupted distance and times are you doing now? Give us some idea of where you are.
Without knowing any more details like what your current average distance is, times, and what you can comfortably swim straight, can't give too much help but some ideas that may work.
If you are sure you can swim 10-minutes straight without stopping, try for swimming 15 minutes, rest 1, swim 15, rest 1, swim 15, rest 1, aside from the warm up and cool down. Keep track of what distance you can get in 15 minutes, and try to keep it consistent.
Just doing distance sets may not be great for endurance. Doing sets of 50s, 100s or 200s on a very tight interval where you don't have much rest, (i.e. it's an interval where you get about 5 seconds rest going at a pace you could somewhat tough pace that you can still maintain for a decent length of time) can also real help build endurance.
One of the best endurance swim sets I know is to swim repeat 100s (either yards or meters) on an interval that would give you about 10 seconds rest. Since I don't know what your swimming background is, it's hard to give you more specific info, but that is a set that's commonly done for endurance. In fact, I swam that as part of my main set today!
I agree 100s with limited rest will do the trick
I also find it helpful for endurance to do 3x200 free trying to negative split each 200 by the 50s you don't have to physically time the set but it is helpful to visualize your speed and try to match that and a little more on each 50. You could also practice the event you are mostly training for in its entirety, say you are doing a 1000 free you could swim the 1000 every day at an advanced pace so you are more than familiar with the swim when it comes time to race it, but it all depends on what race you are training for and how fast you want to swim it.
HOPE THIS HELPS