I also like to load up my pack when I go for workouts. Last summer I got some pretty weird looks while busting my hump on the inclined treadmill with my Eberlestock bag with a sac of salt in it (55lbs). I start wearing that bag for all my walks and other workouts that I can fit it into. Every night I go out for a 3 or 4 mile walk with my bag on. Towards the start of the season I ramp it up every week. The week before I went goat hunting last summer I was doing 17 miles a week with 70lbs on me after dinner.
I'm at about 2500ft where I am and I don't find a noticeable difference when I go to the big mountains. Being high for my training has really been great for me.