I just got back from my first master's swim practice in years. I have been doing most of my training on my own, and I have been feeling a little down on myself and the shape I'm in since the half marathon. It was fun to get in the pool with other people and realize that I can keep up.
I opted to swim with the beginners, because I haven't been in a pool since August. I still think that was a good choice, but I have hope of being "promoted" to a faster lane. I went with my friend and triathlon training buddy, who is a former competitive swimmer. She still has that V-shaped swimmer's build and probably is faster than most of the people at the more intermediate practice, but like me she's been out of the pool for a while, so her endurance is lower.
One thing cool about all the running I did this summer is that I kept my endurance. I finished an hour practice feeling great, other than a slight twinge in my right shoulder telling me that I worked it. Hopefully by the next practice, I'll have my heart rate monitor and can see how it works in the pool.
If you're thinking about a triathlon and you have some previous swim experience, I highly recommend finding a master's swim group near you. Training on your own is OK, but it helps to have a coach and some teammates to help push you a little.