I would start out nice and easy, you would be surprised how hard it is to do a lot of continuous laps even at a slow pace if you are not accustomed to doing it (feel it more in your traps/shoulders than actually feeling tired) I would reccommend starting with 50's (down there and back) make sure you stretch out your stroke, do a good warm up before you get in (stretch arms, legs, chest etc) and once again when you get out to avoid being too sore after a tough work out. Do the 50's on like a minute, if it is too slow pick up the pace, I don't really know your skill level, so times may need to be adjusted. It gets a bit boring, but twice a month we would have 10k nights, 100 x100yards on the 1:05 for me (couldn't do a flat minute for more than like 60 or so) You could work up to something like that, you will have to find your pace, also find your ideal heart rate, and try to keep your rate in that area, when you are working hard you can just put your hand on your chest and feel the beats, count them for 6 seconds using the lap clock, and multiply times 10. If you want to mix it up some once you get into some shape, do a few sets of 1x200 40 seconds rest, 2x 150 on 30 seconds rest, then 2x 100 on 20 seconds and 4x50 on 10 seconds, pushing harder throughout the set, so the 200's are really working your stroke, more than speed, the 100's are getting towards race pace, and the 50's are 100 percent race pace. Oh almost forgot, if you want to get into better aerobic shape, the sprints will help, but what really helps is hypoxic breathing. Do like 10x100 with 30 seconds rest, pace doesn't matter, but do the first one breathing every third stroke, then fifth, then 7th, then ninth, then 7th, then 5th, etc. It is important that you alternate breathing sides or else you will get a "stitch" which is a really sharp pain that feels like a cramp on your side. You will have a headache after doing these but that is just your brain learning to deal on less oxygen.