When I hike over rough terrain, with altitude changes, I do 15 miles in about six hours with a 30 pound pack. On flat terrain, I would add another five miles. Unburdened, another five. More weight, more mountains, less time, weather (snow, rain, cold) would slow me down and I'd make less distance. I am going on a four day hike next week at seven to ten thousand feet. We are planning on ten miles a day because of the cold and altitude. Again, this is six hours a day, with a hot breakfast, stopping to rest at lunch. If I were the protagonist of your story, I might have a reason to go faster.