They start you out slow. The first road march while be all your deuce gear and your pack only weighed down with only 15-25 pounds and the pace is pretty slow and the distance is pretty short. By the end of boot camp you will be in almost full battle rattle and a 50-60 pound pack and the pace is pretty quick and you will probably cover 15 miles.
But those loads, distances and pace in boot camp ain't nothing when you go to MCT or SOI. You are in complete full battle rattle with a full pack of around 80 - 90 pounds. I think I weighed it once and it came out to be about 85 pounds. Plus you have to carry 6 magazines fully loaded with blanks and tripods if you are on a crew served weapon. I would not want to be a mortarman carrying around that heavy base plate, tube and ammo for that thing. Even if they split up the components amongst several Marines. Sucks. This is where you learn to take only what is needed and what they tell you to bring. I remember during MCT shooting off as many blanks as I could as quick as I could so I wouldn't have to hump that weight around anymore.
Pretty much on one road march, we started right before sunrise and ended right before sunset, taking breaks every couple of hours depending if we were on schedule or not. They said we covered around 28 miles. MCT- Camp Geiger, North Carolina, pretty flat ground, only a little hilly. The sandy trails sucked. I only did one hump in the mountains and that really sucked.
I felt like more of a pack mule than a fighting machine when we did road marches which is a misnomer because it was never on road and we didn't march.