My experiences as an army infantryman in Afghanistan confirm that yes; you can indeed go long distances with a heavy pack! Just remember you gotta hydrate or you'll diedrate (something I heard from a lift operator in Colorado and have since adopted as a personal motto). Don't kill yourself with it though. Rest when you need to rest, and take good care of your feet! Eventually your feet will be tough as leather, but until then I'd invest in some moleskin.