So today I did a full mile. Mostly on an old logging road covered with leaves and dead limbs. I think my strategy will be to keep doing the mile with the 13.5lb pack until I get to 15 minutes. Then I'll up it to 25 lbs and go until I am back to 15 minutes. Then 40 lbs, then 50 lbs until I get to 15 minutes for the mile. Then I'll increase my distance, staying with the 50 lb pack.