On Monday, more than 20,000 ath­letes of all shapes, sizes and skill levels will lace up for the 115th Boston Marathon. We asked Grayson Kim­ball, a part-​​time lec­turer in psy­chology at North­eastern Uni­ver­sity, to explain how run­ners can over­come the acute mental chal­lenges of the 26.2-mile race.

Kim­ball, the pres­i­dent of sports con­sulting firm GTK Sports and a vet­eran of six marathons, is the author of “Grateful Run­ning,” a book that offers run­ners psy­cho­log­ical strate­gies for embracing the rolling hills, unpre­dictable weather and phys­ical stress of the big race.

What tips do you give run­ners who are wor­ried about com­pleting the race?
I preach the impor­tance of pos­i­tive thoughts. I’ll often give run­ners what I call “pos­i­tive self-​​talk” sug­ges­tions, which might not focus on the race at all. The runner could ask him­self, “What would I do if I won the lot­tery?” and then spend the next couple of miles thinking about how he’ll spend $25 mil­lion. This takes the mind off the pain and anguish of the race and focuses it on some­thing pos­i­tive and happy.

Thinking pos­i­tively affects the way you feel and the way you perform.

What is the most daunting stretch of the Boston Marathon?
The most men­tally taxing por­tion of the race is the first mile. Run­ners are pumped up and come out way too fast. I always tell ath­letes to use a trigger phrase, such as “slow and steady.” The more they say it, the more likely they are to employ the technique.

Another chal­lenging part of the race is between miles 16 and 19, when run­ners begin to approach Heart­break Hill. Run­ners are so focused on con­quering the Hill they aren’t paying atten­tion to the fact that they have just come down a steep hill and are going over a bridge — and they burn out.

I advise run­ners to develop self-​​awareness, and ask them­selves key ques­tions every couple of miles, such as “Do I need water? Do I need to slow down?”

What should run­ners think about during the race?
Run­ners should vac­il­late between asso­cia­tive and dis­so­cia­tive thinking. Asso­cia­tive thinking is when a runner is locked into how his body feels. Dis­so­cia­tive thinking is when the mind turns off the body and focuses on, for example, how many blonde run­ners he can spot. If you try to think about the same thing for four or five hours, you will go absolutely crazy.

For me, I put on the Grateful Dead, and I’m gone. I don’t think about how my body feels for the next three miles. I’ll check back in again, and start talking to the guy run­ning next to me.

Every runner finds his own mental zone, but the fewer stressful thoughts you have, the better your run will be.