Good to know, thanks!It doesn't matter if it's in the bag or in the load sling. Just keep it tight to the frame.
Be smart about pack training. Start out relatively light and build strength in your legs and core before you go trying to pack around 80-90 pounds. Rock salt or wood pellets work great.
Your best approach is going to have a lot of diversity in your training. It absolutely should have some strength training involved, some higher intensity interval training (work into this depending on your level of fitness), and some lower intensity/longer duration cardio (this is a good one for a longer hike with a 40# pack).
Plyometrics are great too, be careful not to overdo them.
Its been proven that Ruck training is easier on your body then running. When you run your putting 6-12 times your body weight on your knees, where with ruck training 2-3 times your body weight. When you train with a weighted pack, you build strength and cardio. IMO this type of training is the easiest to help those of us who live where it is realitivly flat. Start out low and work your way up. Last year I was training with 60 pounds in the pack, and it became very easy. Throw some hills in there and that will help work on your balance with an offset load on your back. Just remember to keep the load and pack tight to your back.Not sure how Marcus does it, but I would advise against training with a "heavy" pack, depending on what your definition of heavy is. That's a ton of stress on your knees.