An exercise for people who are out of shape:
Begin with a 2 kg potato bag in each hand. Extend your arms straight out from your sides, hold them there for a full minute, and then relax.
After a few weeks, move up to a 5 kg potato bags.
Then try 25 kg potato bags, and eventually try to get to where you can lift a 50 kg potato bag in each hand and hold your arms straight for more than a full minute. This is the level I am right now.
Once you feel confident at that level, put a potato in each bag.