If slowing metabolism increases longevity, how come exercise, which increases metabolism, is beneficial?

I think that's a really good question.

I can throw out a few ideas, but they are somewhat speculative:

1. Perhaps one can analogize the human body to a car. If you drive a car all the time it will wear out quicker. But if you leave it sitting in your driveway, it's also bad.

2. If you increase your metabolism through exercise, at least you don't have the problem of extra energy sitting around in your body which can be used through mischief.

3. A lot of exercise really does wear out your body quicker, but the benefits outweigh the harm.

4. The increased metabolism from exercise is primarily focused in your muscles which can handle it; muscles are not prone to cancer.

Edit: Oh, and of course Number 5: My hypothesis is dead wrong.

As an endurance runner with a BMI of ~20 on an eat-as-much-as-you-like diet, is my calorie consumption is optimal for longevity?

Since I think mild restriction is better than ad libitum eating, I would guess "no." Although your low weight suggests that for you, ad libitum eating is somewhat restrictive anyway.