Thousands put their personal endurance to the test Sunday in Castaic, moving through the Merrell Down & Dirty Mud Run.

Part of a nationwide tour that stopped in Castaic, more than 4,300 runners climbed, slid and ran through a large four-lane mud pit through slippery slopes that led them to the shores of a frigid Castaic Lake.

“We do a huge incline, and the first three miles are pretty brutal,” third-time participant Alec Pedersen said.

“It’s kind of like life — if you get through the hardest part, it’s downhill, and it was beautiful,” he said.

“Getting to the top of the hill, seeing this morning how beautiful it is with the lake and being out in nature — it’s great to get up and get out,” he said.

The race is a test of human endurance with more than 20 deterrents punctuating the course.

Runners were required to conquer obstacles ranging from “monkey crosses” to “Marine hurdles” to cargo-net climbs, as well as battle any sense of apprehension that might deter them from finishing.

For first-time competitor Sheri Barke, a sports and wellness dietitian at the College of the Canyons Student Health Center, the conditions were, at time, nerve-wracking.

“I’m nervous about the mud and the cold lake water, not so much about the obstacles and running,” she said.

“I’m turning 40 this year, so I’m doing something crazy to mark that,” she said.

“The obstacles were fairly decent,” said 21-year-old Valencia resident Zach McGraw.

When asked what his favorite part of the run was, McGraw smiled and replied, “The inflatables.”

“You get to climb up an inflatable like a kid and slide down at the end,” he said.

For a majority of the more than 4,300 participants, accomplishing a personal goal was one of many reasons that drove them to compete, another being the will to donate funds to a worthy cause.

The entire event is a fundraiser for Operation Gratitude, which annually sends more than 100,000 care packages to deployed U.S. service members.

“So far, we’ve raised over $10,000 just with the Los Angeles area,” said event spokeswoman Alicia Bryant.

“It’s a great event and a great way to raise money for some charities,” Pedersen said.

For one runner, coming in first was the culmination of three-years of training and personal disappointment after finishing in second place on three different occasions.

Jose Luis Derman Ramirez came in first place for the 10k race.

Jose Luis Derman Ramirez crossed the finish line first, a few seconds past 46 minutes.

“The experience is great, because you feel the burn in your legs,” Ramirez said. “The pressure is a good pressure. It’s not a bad vibe.”

Ramirez prepared daily by running with varying difficulty and setting goals.

“I run up a mountain in Veterans Park, two hours at the most,” he said.

Ramirez runs up and down hills in the Los Angeles basin every other day and conquers even terrain when he has the chance. With every step, bead of sweat and mile, he believes in pushing his personal limits.

“If I run 10 miles in a certain time, the next day, I know I have to run those 10 miles in less time,” he said.

When asked what his inspiration was, Ramirez answered without hesitation.

“(My dad)’s the one that always makes himself present. I like his support; he’s always there and, today, I’m happy he’s here because after three years, he witnessed me coming in first place” he said.

“It means a lot,” he said. “It means that I’ve worked hard and it has paid off.”