When I started, they originally were going to train me how to load a mall route by starting me on 1 24 foot box truck. Not even 10 minutes in, I got moved to a 3 car set, which wasn't too bad. They kept me there for a few days, then moved me up to another 3 door set (but twice as hard) on the same belt. I've since been moved around a few times (this has been a year and a half now) to splitting with 1-3 cars, depending, to doing 1100 pieces (2 post office, Wal-Mart & Home Depot), to a 4 door set with a mall route on it. They'll keep moving you around until they find a spot that suits their needs that doesn't cause you to be too far behind and cause issues.
You'll get the hang of it. Learn to work the belt, it's the best thing. You don't have to hustle to get it done, just work ahead as much as possible. If you're looking at boxes at the back side of your doors, stack everything on the floor and start again at the front. Work the belt there and catch up slowly.
Don't sweat the load charts that they give you 20-30 minutes into shift. If you're in the same spot, you'll know how things are going to run that day just from loading them in the past. Half the time the charts aren't even right.