Here's the truth:
Package is the hardest classification, period. You will work your butt off in the heat, wind, rain, snow, cold, day and night. You will run, sprint, jump, carry, push, pull and will yourself to complete your day in an incredibly small window of time, in proportion to your load. However, it can be the most rewarding as you will wear the shield and don your browns as the face of UPS.
If you are banged up now, you will be broken in a few years. Don't put yourself in the situation where your production and value as a driver becomes a liability to your center, thus putting your job in jeopardy. Not everyone gets to play in the NFL, NBA and MLB, and the same goes for being a package driver for UPS. You know right now whether this job is a fit for you. There are plenty of other career paths at UPS that will lead to substantial income. Don't get hung up on driving. Good luck to you.