There's a number of guys still driving package cars in their '70's. It can be done. Of course by then, you get the easiest route available.
As for running on the side? That's crazy to do as a UPS driver. Running is one of the highest impact things you can do to your knees. It's fine if you don't do a job like ours, but you're just compounding the likelihood that you'll need knee replacements. If you want to make it to retirement, don't run.
I'm just 34 and my knees are feeling old already. Kinda concerning.