A few years ago I coached a runner who was training for her first marathon and who really struggled with her long runs. Every Sunday, without fail, Claire would fade in the late going, slowing down involuntarily and feeling lousy. The first time this happened, I dismissed the fade as just one of those days. When it happened again the following week, we both blamed the weather. But as the pattern continued, I knew there was a problem.

It seemed to me there were two possibilities. One was that I was asking Claire to run farther than she was ready to. But this seemed unlikely to me, as I was actually being rather conservative in extending her range. The other possibility was that Claire was simply running too fast, and a closer inspection of her pace data confirmed this was the case. So I asked Claire to slow down, but she faded yet again in her next long run and in the one after that.

woman running strong long run
Photo: 101 Degrees West

Then I got an idea. With another Sunday approaching, I imposed a special rule on Claire: the last mile had to be the fastest of the entire 16-mile run, even if by only a second or two. It worked. Knowing that delivering on this requirement would be excruciating at best and impossible at worst if she paced the run as she had her previous long runs, Claire started off more cautiously and was able to finish strong.

Running to Failure

Prior to this breakthrough, Claire asked me if it was normal to slow down involuntarily near the end of a long run. My answer was “Yes and no.” Bonking in long runs is normal in the sense that it happens quite commonly, especially among less experienced runners. But it is not normal in the sense that a runner should accept the phenomenon as unavoidable or benign.

Exercise scientists use the term “failure” to refer to exercising to the point where an involuntary decline in performance occurs. If it were beneficial to go to failure in long runs, then elite runners would do it routinely, but in fact they seldom do. The consequences of going to failure are twofold. First of all, any running you do after the point where you’ve begun to slow down without meaning to offers no benefit. Your body can absorb only so much training stimulus in any single run. When you get to the point where you can’t help but slow down, your body has absorbed as much stress as it can successfully adapt to that day. After that you’re no longer training, you’re just punishing yourself.

woman tired on long run
photo: 101 Degrees West

The second consequence of fading in long runs is that it increases both recovery needs and injury risk. Running form tends to break down at high levels of fatigue, and when form breaks down, tissue damage accumulates in the muscles, bones, and connective tissues. Even if this damage doesn’t result in plantar fasciitis or runner’s knee or some other overuse condition, it will leave you less ready for the next run.

Since working with Claire I have applied the “fastest mile last” rule to many other runners who struggled with long runs, and it has been met with consistent success. For whatever reason—perhaps because it scares the runner, or maybe because it gamifies long-run pacing—it works much better than simply telling the runner to start slower. If you find that you consistently lose steam in the later miles of your long runs, give this rule a try and see if it doesn’t work for you too.

Practicing the Fastest Mile Last Rule

The first step is to rule out the possibility that you are trying to go too far in your long runs rather than too fast. If your long runs account for more than half of your total mileage in a given week, or if a given long run is more than two miles longer than the longest run you’ve completed in the last three weeks without fading, then you may indeed be pushing your long-run distance too aggressively. Otherwise, it’s very likely that you are indeed running too fast.

To find out for sure, enter a recent race time or time-trial result into my online 80/20 Run Pace Calculator. If your pace in these runs exceeds the tool’s recommended Zone 2 pace, you have your answer. In this case, your next step is to plan a long run of appropriate length in which you not only keep your pace within Zone 2 the whole way through but also keep the fastest mile last rule actively in mind, fine-tuning your pace within this zone by feel to ensure you have something left for that last mile.

fastest last mile splits

The goal is not to blast your final mile as hard as you can. If you execute the run correctly, your last mile will be no more than a few seconds faster than your fastest preceding mile and it won’t be especially hard. It’s okay to err on the side of caution and start the run at a pace that turns out to be slower than necessary to ensure you finish strong. Nor is it the end of the world if, on your first attempt, you screw up and fade at the end of the run despite trying to obey the fastest mile last rule. Over time you will move steadily in the direction of being able to complete your long runs at a fairly steady pace that is neither needlessly conservative nor a set-up for a late fade.

A Tool, Not a Crutch

Once you’ve gotten the hang of long-run execution, you may graduate from the fastest mile last rule, leaving it behind and no longer fussing over the numbers. From that point on, it’s good enough to know you could have picked up the pace in the final mile of each long run, whether or not you actually did.