It needed to be refueled right after takeoff, before it had gone high altitude. In part because it never took off with a full fuel load (to limit strain on the machine and risks in case of takeoff flameout), in part because the fuel lines were literally leaky on the ground (to allow for thermal expansion, and because the fuel was used as coolant and fuel lines ran throughout the machine) and in part because the J58 had been designed specifically for high-speed high-altitude operation, they got more efficient with both speed an altitude as they switched from turbojet to ramjet regime. Sub-sonic SR-71 burned fuel like nobody's business to say nothing of takeoff itself.