I've done quite a few sandbag runs, with 50# and 100# bags.
4 miles was my longest with the 50# bag. During those 50# runs, I do intervals where every .25 or .5 mile I stop running and do 10 squats or cleans with the bag. Nasty workout! More than anything it builds mental toughness. Hill sprints with a 50#er are also great.
Typically, I carry the sandbag behind my neck. On occasion, I shift it to be on one shoulder, or even hold it like a football for a short time. It's only 50#s, but the weight takes a burden on your shoulders.
The pain and fatigue that occurs at the site where you place the bag (i.e., shoulders, etc.) is easily the biggest challenge of sandbag carrying.
Now, at 100#s things start to get really painful! After a short time, it's VERY difficult not to drop the bag. I've done runs up to 2.5 miles and boy do they hurt! I think my PB for a mile was somewhere around 17 minutes.
Which brings me to my last point. That Rutman mile sounds nearly impossible. If you can do that, you're an absolute animal. I estimate that I would drop the bag AT LEAST 10 times. That means once you factor in the penalties, I would have to finish in 12:30!
BTW: happy to be here!