Just may it isn't the “100 pounds”
1.) Being in good shape
2.) Having a good pack frame designed for heavy loads (most commercially available pack frame ARE NOT)
3.) Knowing how to load the pack frame depending on the terrain to be traveled
4.) Resting before getting tired AND at regular intervals
5.) Selecting rest locations and a pack frame that will allow standing without having to “muscle” the pack on to the back from a knee.