So I bit the bullet and got a new watch.

I already have two GPS running watches, both of which work fine, but neither of them has particularly long battery life. One can run for about eight hours, the other only four. That’s enough to get you through a marathon, but not a 100-miler. Four hours isn’t even enough to get you through a quarter of a 100-miler, so I couldn’t even alternate watches.

At one point, my idea was to wait until the school year began, then ask my students, specifically those on the cross-country team, if they had a decent GPS running watch I could borrow for one weekend. Not the most elegant solution, but it’s free. Then I saw a GPS running watch with a 13-hour battery going for half-price. Eh, what the hell.

And I love it! It’s fairly similar to my other two watches in that it doesn’t have a ton of features I’d never use. Despite the much longer-lasting battery, it’s about the same size and weight (I don’t see the appeal of wearing a quarter-pound watch that’s the size of a northeastern state). Since there’s not a bunch of unimportant data cluttering the screen, the numbers display nice and big.

Naturally, I got the women’s version. The only difference? It’s smaller. Well, so am I.

One feature the watch has, which I’ve never had before, is a heart rate monitor. It’s supposedly not quite as accurate as the ones that use a sensor on a chest strap; this one is on the watch itself and takes your pulse from your wrist. I’m not that interested in using it frequently, but I was certainly curious, so I made sure I could switch back-and-forth between looking at time and distance vs. heart rate and calories.

What I found was surprising!

1. While I run, my heart rate is much higher than I expected (around 170!)

2. Despite that, I somehow burn less calories per mile than you’d expect based on my age, height, and weight.

3. Perhaps most surprisingly of all, my heart rate drops when I run up a big hill.

I have no explanation for any of these. #2 sheds some light on a question though.

A couple weeks ago, my long training run was a marathon in distance, and I burned only 2,100 calories. Considering I was running in my hilly hometown, in summer, in Texas, that probably added to the total, and if I were running a marathon in “normal” conditions, I might’ve burned less than 2,000 calories.

As a rule of thumb, most people burn roughly 100 calories per mile, which would come to 2,620 calories for a marathon. The human body can only store about 2,000 calories worth of glycogen (ready-to-burn energy), which means 20 miles into a marathon, many runners have burned through their energy reserves and “hit the wall” due to a lack of fuel.

As a little guy, it appears I burn roughly 2,000 calories, perhaps less, in the course of running a marathon. Is that why I’m especially good at that distance? I’m not particularly fast at any shorter distance, and in my limited experience running ultras, I’m good-not-great. Maybe marathons are my thing because that’s the only distance at which most people hit the wall, but I barely don’t.