Kingfisher wrote on Mar 11^th, 2011 at 5:34am:

I load mine with 30lbs of sand and set off on a fairly rugged trail. There is definitely some tenderness across the shoulders the first few times until the area toughens up for the season.

I use the same idea, except I use left over plastic water bottles (refilled with tap water).  Its a bit cleaner than sand and you can add in known increments of weight.  And if a water bottle springs a leak ... well, its just water.

I started carrying a pack on my walks beginning March 1.  I still do the same 5 1/2 - 6 miles and try to keep the same pace as before I added the pack.  Its usually 1.5 - 1.75 hours for my route, which is mostly over mountain biking trails--narrow, lots of rocks, some climbing/descending, etc--this is about as rough a trail as I can find in my area.  And to help you keep your focus, you have kamikaze mountain bikers headed right for you that you have to step out of the way of.

dd