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.