I recently did this, which is from one of Dan John's DVDs:
You need: Sea sack, sled, farmer's handles, plates.
Put on the sled with harness or attach the sled attachment to your weightlifting belt. Put some weight on the sled. Put on the seasack, filled with sandbags. Pick up the farmer's handles (with added weight if necessary).
Walk till you drop.
This works your core pretty well, but also other stuff
