Taking a lap or two around Lake Merritt on a weekend morning is an East Bay must-do. The bird-watching, the people-watching, the city reflected in the lake: It’s hard to beat, and impossible to replicate. But what happens when the 3.1-mile path around the lake transforms from a meandering pedestrian thoroughfare to a long-distance runners’ circuit? Is the water as peaceful, the trail as charming, the shade as nice? There’s only one way to find out: Spend a day running laps. That’s precisely the goal of the Lake Merritt Half-Day, which is a bit of a misnomer because the “half-day race” is a mere six hours, while the main event is a twelve-hour, continuous race around the lake. In both cases, participants begin at 7 a.m. and run as many laps — or miles, as it were — within the allotted time. The men’s records are 76.3 miles for twelve hours and 40.4 for six; women’s are 67.9 miles for twelve and 37.3 for the half-day. Think you can beat ’em? Better yet, think you can survive? There’s only one way to find out.