First of all, the name: Bart Yasso is the chief running officer at Runner’s World. I have no idea what that entails, but Bart has been running for something like a century and a half and I’ve known him just about that long. He’s run a zillion marathons and done all sorts of wacky running stunts (such as running across Death Valley to the top of Mount Whitney in July and doing another race nude). If you’ve ever been at one of the major city marathons, you may even have met him at the Runner’s World booth. Chances are he even remembers your name from another race.

But one of his more remarkable feats has been dreaming up this marathon workout which is also—in a roundabout way—a predictor for marathon success.

Essentially the workout is nothing more than running 800-meter repeats. Nothing innovative there. But when you dig a little deeper, you see the genius behind it.

The gist of it is: By running 800 meters 10 times with a decent recovery, you can get a good approximation of what your marathon time will be. That’s right. This workout—when done correctly—will predict your marathon success. (He and I used to do this workout for years and while it’s worked for a lot of people in terms of predicting marathon times, it doesn’t work perfectly for everyone, including me.)

Anyway, for an example, according to Yasso’s formula, if you can run 10 x 800s in four minutes each, you will run a four-hour marathon. Simple. Get your times down to running the 800s in an average time of, say, 3 ½ minutes and you will run the marathon in 3:30. And so on.

It makes sense because Yasso 800s incorporate speed, endurance, a sense of pace timing and mental tenacity—all important elements for running a quality marathon.

The way to do Yasso 800s is three months out from your marathon begin a weekly workout of running 800s. You can do them on a track or measured length of flat road. A trail is also fine if it’s flat and not too gnarly. Do a proper warmup before attempting the 800s.

Your first week out run two or three 800-meter repeats. Don’t try to run at top speed. Merely run the 800s at a comfortably fast pace. At this stage, don’t worry about the recovery time. Give yourself adequate time to recover by doing a slow jog before launching into the next one.

The following week, add another 800 to the set. Every subsequent week, add one more 800 one until you can run 10 of them in one session. Find your speed level as you go along. Your speed should be about your 5-K race pace. There shouldn’t be a huge difference between the first 800 in a set and the final one.

Try to maintain a smooth, even tempo. Work your arms, run with good posture and aim for a quick leg turnover. You aren’t training for the 800, but the marathon and this speed will pay dividends in the race by making you smoother and more efficient.

The recovery jog between each 800 will get shorter and shorter as you get fitter and faster. But it should be approximately as long as the previous 800 took to run. That is, if you ran the 800 in four minutes, take a four-minute jog recovery. Don’t cheat and take a longer recovery.

You’ll get faster with each week. When you can finally do 10 of them, your times for the 800s should be in a fairly close time range. You don’t want a bunch of them real fast and the latter half much slower. You’re aiming for consistency. Average the time for the 10 and—voila!—this should be your marathon finish time.

Obviously, it will only work if you are healthy on marathon morning and have done all the other proper training components, most importantly the long runs. It doesn’t always work out exactly perfect as a predictor—there are plenty of other factors that may come into play such as the terrain and weather—but it’s amazing how often doing 800s in this manner is a good indication of what your marathon time will be.