To squeeze the toughest aerobic workout into the least amount of time, one need consider only one word: climb. Gravity, and your aching muscles, will take care of the rest. From the base of Mount Diablo at the Athenian School in Danville to the parking lot at the summit is 3,249 feet dispersed over 11.2 miles of winding asphalt. That’s an unrelenting 5 percent grade — not deadly, but enough to make you think. Since 1981, bicyclists from throughout the Bay Area and beyond have accepted the Mount Diablo Challenge; this October, in the race’s 29th year, more than 1,000 riders are expected. Finish times range from the mid-forty-minute range to more than two hours, and riders come from a diverse range of backgrounds including recreational weekend riding, team racing, and just about everything in between — including, these days, fixies and unicycles. It’s all for a good cause (beyond burning lungs and calves), as the nonprofit that organizes the event, Save Mount Diablo, also benefits from proceeds. The only thing better than the view at the top is the ride back down, and our old friend gravity helps out there, too.