On April 24, eight days after American running star Galen Rupp dropped out of the Boston Marathon in the 20th mile with hypothermia and breathing problems, organizers of the Prague Marathon announced that Rupp had been added to the start list of their event, to be held May 6, a day shy of three weeks after Boston.

When I saw this news I thought, ‘I can relate.’ I’ve come away from several disappointing marathons hungry for another try, and on three occasions I have acted on this hunger. Indeed, the phenomenon of the “bounce back marathon” is quite common, and understandably so. It takes a long time to prepare for a marathon, and there are so many things that can go wrong on race day that it’s unsurprising runners are often tempted to redeem a poor performance—whether it’s due to unfavorable weather, GI issues, or whatever—with a quick next marathon instead of sticking to the original plan of taking a break and starting a whole new training build-up. But are bounce back marathons a good idea?

It depends. Recently, an athlete I coach performed below his expectations in a marathon due to an ill-timed health setback that prevented him from eating anything on the day before the day before the race. Afterward, he told me he wanted to do another marathon as soon as possible in order to “take advantage of [his] fitness.” I talked him out of the idea, saying it was too risky. Subsequent events revealed this to be sound advice. Even after a week off followed by a week of very light training, this runner felt sluggish and beat-up during his runs and it took him a couple more weeks to get his feet back under him. If he had attempted a bounce back marathon instead of taking a break, it would have been a disaster.

As a general rule, attempting a bounce back marathon is a bad idea if A) you truly peaked for your last marathon (that is, you trained pretty much as hard as you could without overdoing it) and B) you ran the marathon as hard as you could and finished it. In these circumstances, your body needs a break, whether you realize it or not.

Two of my own three efforts to get right back on the horse after a disappointing marathon ended in injury. After the 2006 California International Marathon, where I aimed for 2:39 and ran 2:47, I returned to heavy training within a week and immediately developed a hamstring injury. Three years later, after the Boston Marathon, where I aimed for 2:37 and ran (and walked) 3:18, I started the Orange County Marathon 13 days later and quit halfway through with a bad case of plantar fasciitis. Only once did I get lucky, after the 2016 California International Marathon, where I aimed for 2:45 and ran 2:58 and 13 days later solo time-trialed a 2:49 marathon around my neighborhood. (Crazy as this was, I must confess it was quite satisfying.)

Bounce back marathons are less risky if you DNF your first marathon for a reason other than injury, as in these cases your body emerges less wrecked than it would be if you’d covered the full 26.2 miles. They’re also less risky if you don’t train to your limit in the cycle leading up to a marathon. I used to wonder how some of the top ultrarunners get away with competing as often as they do. Then I trained for a 50-miler and realized it’s because the body doesn’t need deep rest as often if almost all of your training is done at low intensity. I ran the Boston Marathon 15 days after my 50-miler and it went just fine because although the ultra itself had thrashed my body, the training leading up to it hadn’t.

There’s a reason nearly all professional runners specializing in the marathon distance run only two or three marathons a year. These folks need to be at the very top of their game when they compete and it would seem that two to three times per year is as often as they can achieve a true peak performance level at this distance, not so much because of that the race does to the body as because of what the training does.

Lately, though, this orthodoxy has been challenged to an extent by a few noteworthy mavericks. Last year, for example, American Sarah Hall placed fifth in October’s Frankfurt Marathon (2:21:21) and won the California International Marathon just five weeks later (2:28:10). And this year’s Boston Marathon was won by Japan’s Yuki Kawauchi, who completed 12 marathons last year, winning five.

I wouldn’t put too much weight on these special cases, however. The most important thing to keep in mind is that bounce back marathons are inimical to the goal of developingas a marathon runner. Although it is possible sometimes to turn around quickly after a marathon and perform satisfactorily in another one, you will not get better at marathoning this way. Developing as a marathoner demands that you take a break after each marathon, intentionally giving away some of that hard-earned fitness, and then start a fresh training cycle. This is the true way to “take advantage” of all the hard work you put into preparing for each marathon.