There was a time not so long ago when I would regularly scan the race results on Cool Running just to stay informed about the local running scene. In those days, it didn’t take long to look through the handful of links, take in the top twenty or thirty runners (most of them familiar to me), and imagine where I would have finished had I been there. I remember that Cool Running was a huge improvement over reading the agate type on the back pages of the Boston Globe sports section, assuming the Globe even had the results. In other words, the local running scene felt like a cozy enclave, where there might be one or two big races every few weeks, and a handful of smaller races sprinkled here and there.

Nowadays, the idea of scanning race results is a lot more daunting. For example, on Sunday afternoon I went to Cool Running to find out how CSU folks had fared in their Pub Race. I found links to results from 30 separate races held on Sunday in Massachusetts. The day before, Saturday, there had been another 16 races. Perhaps, I thought, this particular weekend in early October is peak racing season and an aberration. Not so. The weekend of September 27-29 saw 48 races in Massachusetts. The weekend before THAT saw 42 races.

OK, so there’s a lot of racing going on, or a lot of running anyway. But the other thing that struck me was how many of these races were 5Ks. On Sunday, the day of the Pub Race in Dedham (a 5K), there was another race — another 5K! — in the same town, and two others — also 5Ks! — in two adjacent towns. I counted up the number of 5Ks over the past month, and it appears that in a country that hasn’t switched to the metric system, approximately two-thirds of all road races claim to be 3.10686 miles long. And that figure doesn’t even include all the high school and college cross country races going on, the vast majority of which are also 5K. How did the 5K distance take over?

Well, 5Ks are short but not too short. For most runners, racing a 5K takes approximately the same amount of time as watching an episode of “The Big Bang Theory.” Coincidence? I think not.

The motivation of the runners is only one factor, though. From the perspective of race organizers, a 5K is an ideal distance if your goal is to attract a few hundred runners, limit the disruption to local traffic, and not have to pay too much for police overtime. The whole thing is over and done with in under an hour, leaving time for raffles, beer drinking, and (if it’s a high-class affair), a post-race DJ. I’m not complaining, by the way. It’s great to get to the end of the work week and say, “I feel like doing a road race this weekend,” and find that there are five races (all 5Ks) within ten miles of your home. It makes advance planning almost entirely optional.

I also think that the rise of the 5K reflects the tastes of casual runners who enjoy the race vibe but don’t really run enough to enjoy longer races. I say “casual,” but that’s a bit of a value judgement. They are serious enough to enter road races (and, in case you haven’t noticed, road races are not cheap).

Speaking of serious runners, a few days ago the Boston Globe had an article about the trend of offering sight-seeing tours for runners. Instead of being on a bus or Segway, participants actually trot along for several miles, stopping now and then for more in-depth descriptions from their running guides. I mention this because the article claimed that there were “19 million serious runners” in the U.S., based on the number of people who register for road races. If accurate, that’s a huge number.

Is there a down side to so many 5Ks? Are they really like kudzu, that Japanese vine that crowds out all other competing species? Probably not. If anything, it’s probably the case that having a few dozen 5Ks every weekend keeps the running scene healthy by getting people who might not otherwise enter races to join in the fun. And the proliferation of 5Ks spreads out the opportunity for finishing at or near the top. The fast guys can only go to so many races. While Tyler was winning in Somerville, and my other buddies and all their fast friends were racing in Dedham, I should have snuck off to one of the other dozen 5Ks — preferably one with no advertising budget or prize money — and run 19 minutes to scarf a cheap win.