I have a collection of 19 dive masks, a fair number of snorkels, and enjoy my pool work with several different types of Fins. I usually take 5 different mask/snorkel combos, and either one or two different sets of Fins, to the pool.
I swim a 25 yard pool, and swim my first set of 200 yards freestyle, sans Fins. I then swim another 200 yards breaststroke, with my first 50 yards very slow to allow my knees to get used to the stroke, then speeding it up on the second 50, going hard on the third 50, then slowing down for the fourth 50 breaststroke.
I then stop and put on my Fins, and change masks. I use different masks each 400 yards so I don't have to count laps (400 yards x 5 masks = 2,000 yards). With the mask/snorkel combos, I usually swim 100 yards just kicking, then 50 yards freestyle with Fins, then the last 50 slower, just kicking a flutter kick. I repeat this, using a dolphin kick for 100 yards, then fast finswimming (hands in front, interlocked arms for streamlining) dolphin kickfor 50 yards, then a slow 50 yards to finish the 400 yards. Some of these swims are underwater about 20 yards.
Then, I change mask/snorkel combos and repeat three more times for a total of 5 cycles. My wife usually also swims, and if she is done and I can go to the deep end, I do so and complete two or three dives to the bottom in the deep end 16 feet deep; it's a competitive diving pool which is 50 meters one way, 25 yards the other. We usually swim 25 yard widths of this huge pool. These dives give me a total swim yardage of 2,000-2400 yards. The underwater swims usually last about 30-45 seconds, for 25 yards or a bit more (depending upon the lane I'm in, I may have to swim laterally a bit to get to the deep water), without hyperventilation. I then swim back on the surface, recovering, and repeat a few times total 3-4 underwater swims in deep water. The underwater swims keep my ears ready for spring/summer dives in the Clackamas River.
By the way, I just turned 74, so age by itself is not an excuse for not working out. My wife and I also walk a lot, and today (Christmas day) we walked 6.3 miles in just over 2 hours. Keep in shape for diving, and the diving is more fun. I also learn a lot about my different masks, snorkels and Fins (I've been collecting these for years now), their capabilities, limitations, and which I would select for open water.
SeaRat