I'm also starting soon.. With so many people here hating UPS and many more reviews online about working for UPS I was starting to feel like I signed up for a poisition in hell. However, I'm assuming hard work at this company does pay off. It is a corporate world and a corporate ladder. Suck it up, work hard, you'll eventually get where you need to go.
When I took a walk through the warehouse I saw that it's a lot of repetative work, no matter where they place you. Whether it be loading or unloading, you will be lifting boxes and putting them down. Then you'll life some boxes, and put them down. You'll probably lift some boxes, and then put them down. Only after lifting some boxes, and putting them down. To spice things up though for the last hour or two into the shift you will lift some boxes, and put them down.
From what I've read online, you're never fast enough and it's up to you how to react. What I plan on doing is busting my ass, and if it's not good enough and I get told I'm not working hard enough I'll ignore it and keep working. (Ignore it if I truly am working as hard as I can. No reason to stress what is out of my control) If you get fired, whatever. You were at the bottom of a ladder that supposably sucks anyway.