In reply to jsmcfarland:
I swam two or three times a week for a year or so as well as going running, but now I just run. While I was swimming I did lose loads of weight, but it may not have been the swimming that did it, and I notice that several regulars at the pool, who went more often than me and who did longer, harder, faster sessions than me, looked pretty overweight and didn't seem to be losing any.
I gave up the swimming because it was so boring and because at my pool I was tied down to very specific times of the week. Also, it wasn't very nice if the pool was busy, as it often was, and I never got any faster, no matter how hard I tried.
Running on the other hand, can be done any time, anywhere, with just dogs to cope with, and with more effort packed into less time than with swimming. If you have somewhere nice to run, even better.