I never "train" with a full pack. It has more to do with my age because I do not want put any excess strain on my knees. Others may not have this issue.

What I have found that sudden intensive "training" with more 2000+ feet of elevation gains, will leave me very sore for about a week. I would never do this immediately before a big trip. If you are not overweight, in reasonable shape, I would just add a little bit of "hills" to your existing walking before the trip.

If you keep to your proposed schedule, where are you getting the time to do 7.5+ miles with a full pack? If you plan on doing this a few hours at 4 mph you may be very stiff and sore after this, and a day's rest will not be sufficient to relieve the pain. On the other hand, if you plan to take a slower pace 2-3 mph, then why not just do an overnight backpack?

I "train" on my first of season backpack trip by going slow the first day and gradually adding miles eacg dat if the actual trip. I expect to suffer a bit at the beginning of the season, and it gets better as the summer goes on.

I also find bicycling better for aerobic workouts, than walking. Over the winter I keep in shape with alternating moderate to easy biking (20 miles) and walking the dog (4 miles). My husband uses a rowing machine.