I swam a 51 second 100 and a 21 second 50 in high school and I can tell you that (for me) required a lot of weight lifting, being 6'3", and an absolutely furious turnover rate and outright psychotic refusal to lose attitude. I'm not sure without the testosterone of being a teenager I could put in that kind of training again. I'm a pretty good short distance runner, getting podium in small local 5ks, but my genetic build is definitely for sprint swimming. Once run distances get long, the muscle mass I put on back in the day to swim so fast (plus all the push ups and pull ups and such from ROTC) damages my run capabilities tremendously. And I just don't have the efficiency gift in running like I had the burn tolerance gift in swimming.
One trick we did that greatly improved swim power and speed was to tie surgical tubing around our waist and the other end around the dive blocks and then try to swim to the other side of the pool. Because of the weight lifting I was doing, I was the only guy on the team that could wrap the cord around the block 4 times and still make it to the other side. Set after set of that is the kind of torture we would put ourselves through to get fast. You know it's been a good workout when you can't use your arms to get out of the pool and have to use the steps... and also get help from a friend to pull you out.
I guess my point is that a lot of runners get disappointed when they can't swim as fast as the fast fishes because the work we put in a while back to get that fast was absolutely insane. People can do it because water is such a forgiving environment. And career runners are at a disadvantage because all the signals that correctly tell you to stop when running because you will injure the F outta yourself is typically the signals in the pool that you're just getting started. When your arms and shoulders and lats get all pumped up and the lactic acid is burning like crazy, you've still got 10 or 20 more intervals of that same thing or worse. And then again later today, and then again tomorrow, twice.
If I tried training like that today again at age 43, my shoulders would probably blow clean off my torso. Instead, what seems to work better when we are older are 20 minute long sets of medium hard and improving form to reduce drag.... and then not worrying about it so much. Short and fast stuff is for the kids. Let them have their fun, it's their turn now. :)
Zen and the Art of Triathlon
Interviews with Jordan Rapp, Helle Frederikson, Angela Naeth, and many more. http://www.zentriathlon.com