Long ago in the dim and distant past, our scout troop used to do a forty mile sponsored walk along canal towpaths. The first year you tried it was always the worst - the checkpoints were never where you wanted to stop. By the second year you learned to crack on and get the ground covered, pick up food and drink and carry it along without stopping and only stop when you wanted to stop. Keep to your own pace too, trying to match someone else will always be a compromise. Think of your hands too, swinging your arms will result in swollen hands, grip the lapels of your jacket so you don't swing your arms to help off-set this. Other replies have covered foot and body preparation but theres no getting away from the fact that it is going to be hard work.
Good luck
Ogri the trog
Good luck
Ogri the trog