I think the problem is that swimming is not a weght bearing activity.  So over time your body's adaptations to the stresses of running on joints, tendons, ligaments, etc. will be lost.  Mixing the two - getting in a couple runs a week - can help reduce or prevent that loss.

Len

Out of curiosity, when do you typically run?

I'm a canadian currently in florida, and I have been running through the summer. My runs typically end at sunrise; there's no WAY this cool-weather gal would be able to go out in the afternoon. Just walking to the car after work is deadly.

I have also had a similar problem lately.  I live in Pennsylvania, but I have been working in Puerto Rico and I have found that my performance is worse in the heat than when I am back home.  I have found that wearing compression clothing helps me from overheating longer than anything else.  I also run in the last 40 minutes of light because it is a little cooler.  Carrying extra water and dumping some on your head from time to time helps too.

I have read quite a bit about the benefits of cross training and have read that it can even improve your running.  With that being said, I have also read that you must still run from time to time to not lose your running legs and performance.  If you don't want to exclusively swim, running in the pool can help with your regular running because of the increased water resistance.