Re: We got any swimmers here?
I was a decent distance swimmer all the way through high school. The Army makes it hard to swim with limited pool availability and awful unit PT, but the Fort Carson pool was close enough to my building to get over there pretty regularly. Since it has been a decade since I swam regularly for exercise, I found that alternating 100s with set rest intervals was really helpful to get me back in the swing of things. Then I bumped it up to 200s, threw in some IM and kick sets and called it good. At most, I was swimming 3000 yards a session, but that was plenty to keep my heartrate elevated the entire time and my triceps swolen for two days.
I am going to see if there is a Masters program that would be compatible with my schedule, but I will probably have to go it alone for the most part.
As for underwater swimming, when I was doing 8000+ yards a day at the peak of swim season, I was able to do a full 50 yards (two 25s) and about half of a second length underwater. Cardiovascular efficiency was probably the biggest factor, since my body was used to making do with reduced oxygen flow. I have always been an alternating breather (three strokes per breath) and I think that helped a lot. So, in addition to JPipes' recommendations, overall conditioning will help a great deal.