How can I carry a lot of stuff around without overtiring myself?

You can't. You can not physically carry around lots of things without getting tired. You can carry it on a cart, that way you don't have to exert yourself that much. Our muscles can only take so much stress before they get tired. Working out may increase your stamina, but even then you will have a limit that you can not go over. Endurance - can be your friend though - slowly increasing the amount can help.
Train, . Exercise, go to the gym and increase you stamina and muscle strength. At you age you can take the bull by its horns...