The sun was breaking through by 10:30 a.m., but not before a slow, steady drizzle waterlogged some 31,000 runners and walkers in the Ukrop’s Monument Avenue 10K Saturday morning. What started as a small run with less than 2,500 people in 2000 has morphed into one of the country’s largest races. It also, unofficially, marks the beginning of spring in Richmond — even in the rain.