Stick to level ground, and flatbeaten paths, so hills or rough ground. Whilst this sort of carry is technically possible, its going to be work, and im imagining your not going to like it alot, head down sweating.
As for the distance 20km per day for 20 days with a pack starting at around 80lbs is murder. Soldiers do this for very short stints 2 or 3 days carry to set up a recon position , but i feel that by day 5 you will be dribbling along, in that cycle of needing to carry more food. Take 60lbs out for 3 days and see what your energy levels will be like after.