The day begins at 4:30 a.m. with a half-mile run to breakfast. Then comes three to four hours of grueling physical training: burpees, pushups, flutter kicks, all high-intensity, anaerobic exercises designed to strain the muscles and sap the breath. Interspersed are hikes and runs up the scrubby California mountains, lugging packs weighing up to 90 pounds — or carrying, fireman-style, a 200-pound buddy.