Core Strength

BenchPress and Squats, Bicep curls and Skull Crushers are all important exercises, but, alone they can't prepare you for hours of groucho walking, climbing, running, stopping, waiting and jumping all with a full load out of gear. You core must be able to support your body and allow it to move and balance with ease.
This takes more than a few situps and crunches. Try adding these a couple of times a week.

The side bridge: hold for up to 2 minutes Swiss ball twist

Hanging weighted twist:
You will be surprised what a situp can't do.