What would you pack in a 70lb ruck?

If your ruck doesn't weigh a total of 70 pounds with all of your OCIE gear in it, you can add a sand bag or two

I carried a 5qt canteen as well as two canteens of water, C-Rats, extra fatigues, TA-50 needed in the field, rain gear, sleeping gear, ammo, steel pot, and a transistor radio.
M-60 weight didn't count.

Unless you’re already rucking 70 pounds, starting out at 70 pounds is just going to break you. Also, you should not be trying to make time with a 70 pound ruck. There is no selection or school where you will have 70 pounds and need to make time. Any event like Ranger School or SFAS when you have that much weight, you will be walking slowly. In Ranger School, your patrol pace will be 1-2 km in an hour. In SFAS you’ll either be walking through the woods on land Nav or walking with your team, slowly, on the team events.

That said, if you need weight, the best thing is to make a sand baby. Several sand babies actually. You should start at a weight you’re already used to and increase weight no more than 10% every month or two.

Remember to walk slow with heavy weight. It’s not your muscles you’re training, it’s your tendons and ligaments. Rule of thumb is it takes the times as long for your connecting tissue to strengthen as it does for your muscles. They also take five times as long to heal when you injure them