I find that mantras can be extremely helpful in running, whether used on training runs or in a race. Years ago, my friend, Robert Bleich, introduced me to a mantra he enjoyed using on steep hills: he would say "the hills are our friends". This helped me shift my thinking away from the discomfort I was feeling and towards the idea that I was benefitting from the incline. One of my other favorite mantras was from a Runner's World article. I used it through my first few marathons, but continue to use it in ultras when I remember to - it is simply "Smooth, Relaxed, Efficient." This helps me think about a controlled, steady effort and good form.
I have been trying to ask my ultra running friends, as well as some elite runners, whom I admire, to share a favorite mantra they have used. Some are funny, some are straight forward, and some may have a personal meaning to that person. Take a look and see if any of them stick for you.

"Find a way." -Liza Howard
"You can cry when it's over with." -Brian Ricketts "You got this." -Michele Genereux
"This is unpleasant, but it's not awful." -Jess Winnett
"When the going gets weird, the weird get going." (Quote is actually from Hunter S. Thompson) - Steven Moore
"This is why I trained, this is why I entered. Suck it up and do the work." -Steven Moore
"Visualize the finish line reeling you in." -Tony Maldonado
"Relentless forward progress" - Amanda Basham
"It's okay to struggle, it's not okay to give up" - Justin Grunewald from Gabriele Grunewald
"It's going to be worth it" - Chris McWatters
"Better looking at it than looking for it" - Joe Prusaitis
"If you lie to yourself long enough, you're going to believe it" - Joe Prusaitis
"Everything I need is within me now" - Jason Schlarb
"Nobody in this race matters but me" - Jason Schlarb
"You've got one more step" - Tanya Espalin
"Flow like water" - Joe Tammaro
"No pain, No Pain" (Rocky IV reference) - Billy Yang
"If you are going through Hell, keep going" (Winston Churchill quote) - Michael Chamoun
"Relentless" - Katie Grossman
"The faster you run, the sooner you are done" - Andy Pearson
"I've suffered more than this and still finished" - Howie Stern
"Always be smiling" - John Sharp
"Never quit believing in the power of your own efficacy" - John Sharp
"Life exists on a curve: the highs and the lows won't last forever" (paraphrased) - Jamil Coury
"Suffering is a gift" - Dylan Bowman
"So much love, so much love" - Dominic Grossman
"Victory over adversity" - Clint Oncken