LATHROP — Upwards of 10,000 aficionados of the latest fitness craze are descending on Lathrop April 21-22.

The Northern California Survivor Mud Run planned for Dell’Osso Farms already has 5,400 pre-registrants signed up at $57 apiece for the 3.47 mile challenge course that involves 16 “extreme” obstacles. The Saturday, April 21, date is already sold-out with 5,000 pre-registrants.

Since just recently adding Sunday, April 22, they have already added 400 more competitors.

“It shocked us,” said Susan Dell’Osso who, along with husband Ron, own and operate Dell’Osso Farms Pumpkin Maze as well as the Holidays on the Farm attractions where the firm Survivor Mud Run is staging the Northern California race in April. “It is amazing they got that many people that quick.”

Almost all of the competitors are from the San Francisco Bay Area and Sacramento region.

Mud run challenges typically draw participants from intense boot camp classes that many gyms operate but are much more extreme. The mud run challenge involves everything from rope walls that must be climbed push-ups, obstacles that must be cleared, culverts and - of course - plenty of mud pits to traverse.

Waves of 300 participants start competing at 9 a.m. each day with new groups starting every 15 minutes.

The entry fee includes a Survivor Mud Run shirt, a free beer to participants 21 and older, survivor race bib, free boulder toss competition, survivor trophy for division winners, and post race refreshments.

The day of the competition there are vendors, a beer garden, food, music, and other activities.

Survivor Mud Run is also enlisting volunteers who will get Survivor Mud Run crew T-shirts.

Participants must be 14 years or older on race day. All participants are required to understand that it is not a normal 5K race and is instead an extreme foot race through rough terrain with obstacles and mud. There are 11 age groups with both male and female divisions.

Participants should bring a change of clothes. Showers will be provided.

Several seasoned mud run challenge veterans suggest participants to try on what they plan to run in well before the race and take a shower and bath in them to see how the clothes fit. Then they say you need to go for a swim and a run to determine what fits and what type of clothing would cause problems. They also suggest that you do not wear contacts or any type of glasses as the mud will ultimately create problems for you.

Dell’Osso said that they are thinking about adding their own vision of a mud run challenge when the 2012 Pumpkin Maze opens in October. That could include offering a less intense course so those under the age of 14 can participate.