You probably have a friend who’s a dating chameleon, someone whose passions and pastimes morph to reflect those of their latest squeeze. You run into a buddy you haven’t seen in a while, and he’s carrying roller skates and flaunting a bunch of new tattoos.

“Yeah, I’m pretty into roller derby these days,” he says. You gently ask whether that isn’t typically, you know, a sport for women, and he tells you that his new girlfriend Siren is into it and she convinced the team to let him be the mascot. Ironically, duh. Then the next time you see him, you find out he and Siren broke up but now he’s really into wearing hats. Coincidentally, his new girlfriend is a milliner.

I’m confident that I’m not like that. When I met my wife, Heather, she was training for the Boston Marathon. She’s big into running, and never did it occur to me that I should join her in that sick hobby. I’ve got stuff to do, like sleep in late. Sure, some parts of marathon culture are appealing, like the carbo-loading and slathering yourself in Vaseline—but the running I could do without.

At some point that changed. Apparently, the process happened gradually, such that I was unaware of my metamorphosis, but I can precisely date the moment of my self-awakening. I was inside a port-a-potty at dawn, a half-hour before a 5K race. A freezing wind was kicking outside, and I realized I was taking my time with the hand sanitizer in order to extend my visit in the cozy plastic stinkhouse. When you find yourself relishing your time inside a port-a-potty, you really ought to question what you’re doing with your life. Another race was held in such a downpour that I ran it while wearing a skin-tight trash bag, because I grabbed a kitchen bag rather than a 50-gallon black one. I looked like an escaped Gollum. This is fun?

I guess so, because I’ve now run a bunch of 5Ks. Every race starts the same way, by which I mean I wake up in the morning and think about how great it would be to not run the race. But then Heather gets dressed and I feel guilty. The next thing I know, I’m trying to drink a small cup of Gatorade without breaking stride, which results in half of it going down my shirt and the other half aspirating into my lungs. Then, a mile later, the two molecules of liquid I managed to swallow have migrated directly to my bladder and I start wondering whether I’ll cross the finish line looking like the Uta Pippig of 5Ks. Also, I’ve paid to do this.

I usually do OK in my age group, but I’d figured that when I turned 40 I’d automatically start placing, since my new cohort might include near-50-year-olds. But here’s the problem: As you get older, the 5K crowd self-selects for psycho runners and fitness freaks. Most of the guys in their 30s were, like me, generally not taking this whole thing too seriously. Then you turn the corner into the 40s and all of a sudden it’s not a bunch of hungover fat guys sweating out last night’s Bud Lights. It’s dudes wearing flimsy short-shorts, energy-gel bandoliers and nipple Band-Aids.

You see a guy dressed like that and you just hope he’s not in your age group. Most of my prerace prep consists of silently guessing everyone else’s age so I can figure out who to undermine. Like, one guy might have a tribal tattoo that he definitely got in Cancun during Spring Break ’95, thus he’s a little older. But that other guy with the Blink-182 T-shirt and subtle flecks of gray in his goatee? I’m gonna flat-tire him on the way out of the gate, really psyche him out.

Of course, sometimes you misjudge. Toward the end of one race, I saw a guy ahead of me who looked like he might be my age. So I pushed hard, gave it everything I had and passed him right at the end. Just beyond the finish line, as I was gasping like a fresh-caught flounder, another guy approached him and said, “Hey, how old are you?” (It is socially acceptable for adults to go around asking each other their ages for 15 minutes after a 5K race. It’s like a new kindergarten class on the first day. “You’re 45 and a half? I’m 39 but my birthday’s next month. So I can say I’m 40.”) The dude I just ruthlessly smoked by three seconds replied: “I’m 65.” So that made me feel good. So did the announcement for overall male winner, which turned out to be a 10-year-old wearing a pink cast. At least it was on his arm.

In that race, Heather came in second overall for women, out of 534 entries. She’s a runner. Maybe I’m still not, but mark my words: My times will get lower. My shorts will get higher. But I’ll be a gracious competitor and even share my nipple Band-Aids. As long as you’re not in my age group. ◆

Think that’s funny? Send unbiased emails to ezra@improper.com.