In a physically challenging way, race director-promoter Rick Kozlowski has hatched a whacky concept to conduct a fundraising “fun” run with obstacles strewn along Mission Beach. He calls it the “Sand Pit 5K.” Trudging through sand mounds, water obstacles and hoops, an estimated 1,000 runners will engage in exhaustive tests from Belmont Pier to Grand Avenue and back. The first of six waves of runners in various age classes begins Saturday, Sept. 17 at 8 a.m. To avoid added congestion, the waves of runners will be spaced two minutes apart.  Participants will run in and out of the surf line, climb over barriers and even jump through hoops en route to a conclusion of two pits and a sand climb. For those not inclined to run the full 5K course, there is the “Sand Puddle,” which is half the distance — but only slightly less challenging. “It all came about last year in a dream I had about people just having fun,” said Kozlowski, who has staged standard 5K runs for 30 years. Entry fees range from $30 to $45. A portion of the proceeds benefits the Mission Valley YMCA. “Five obstacles are on the ocean and hard-pack sand side as the runners go north.” Kozlowski said. “After a U-turn, they’ll return on the soft-sand side and face another five obstacles. “At the end will be four more obstacles. There will be a 20-foot-long sand trench, a 10-foot-high mound climb and a four-foot sand pit filled with water. The last is a treasure hunt where contestants pick up containers that might have a prize.”  He said the average run will take about 40 minutes and about 15 minutes for The “Sand Puddle” run.  Awards will be given to the top three finishers in each age group for the “Sand Pit 5K” and to the first-place finisher in each age group for the “Sand Puddle.” For more information, visit www.active.com/running/san-diego-ca/the-sand-pit-5k-fallmania-2011.