Bassplayer,
My two cents here. I would not increase my exercise by more than 10% per week for any given activity; you can use either time or distance. I would also highly recommend cross-training, bike and swim, bike and walk, elliptical trainer and swim, etc. I think it helps if you can find something you love to do, then it does not seem as much like "work" nor is it as hard to make yourself do it. Joining a group can sometimes help, like a walking or hiking group, or a cycling club, or master's swimming. But I think cross-training is very important so you work different muscle groups and that can help with injury prevention and overuse. As far as HR monitors go, I love my Polar for the pool as it is highly reliable but prefer a Garmin or Motoactv for other fitness activities. I like having the GPS capability for when I bike outside. My Motoactv has actually become my favorite because it is also an MPS player but when Google bought out Motorola they discontinued production of it, but I use it for everything other than the pool. Unfortunately, only the analog watches work in the pool, not the digital ones.