Asimo ain’t got nothing on this bot. The Cornell “Ranger” robot was designed for walking long distances, and so it has done, achieving a world record of 5.6 miles — 45 laps around their running track (movie in the first link). Eventually it stopped and fell down, possibly due to a low battery.
That’s quite a long way to go on a charge, though; the BigDog has a gas motor and Asimo I can’t recall being shown for more than, say, 45 minutes at a time. It’s efficient because instead of using servos and actuators (or whatever they’re called) to modulate every movement of its legs, it walks like we do, by using gravity and momentum to keep energy costs low. Congratulations to the team!