This happened to me. The most important thing that changed when I discovered I was working out too high is I suddenly didn't feel exhausted after my workouts, didn't feel the need to nap . . . actually felt energized. The weight loss was an added plus.
I wear my heart rate monitor now whenever I cycle, spin, or play tennis. I don't wear it when I swim, but I know a lot of triathletes who do. It has been a really great way to monitor my resting heart rate . . . and fun to see that go down as I've gotten into better shape. Not everyone enjoys it, but I love mine. Also, after I convinced a friend to get one, he lost 50 pounds (over 2 years) after working out for years without losing.
Sandy