LATHROP — It’s billed as a challenge but it is really one big muddy day of fun.Some 10,000 competitors plus an estimated 3,000 spectators are invading Dell’Osso Farms today and Sunday for the Northern California Survivor Mud Run.Organizers were busy Friday putting in place the 16 “extreme” obstacles on land that every October is home to a 20-acre corn maze. There are 5,000 participants today and another 5,000 on Sunday. Waves of 300 participants start competing at 9 a.m. each day with new groups starting every 15 minutes to cover the 3.47-mile course.Mother Nature is cooperating with the high today expected to hit 88 degrees while the top temperature on Sunday should reach 87 degrees.Most of the competitors are descending on Lathrop from the Bay Area and the Sacramento region.