Do it. Anything over 30-something pounds is a two-man lift.

Wear a back brace, buy one yourself if not issued one.

I think at the end of it all you'll be in much better shape. You may want to prepare by doing squats and lifts and probably some cardio. At age 50, with average health, I'd expect your biggest issue to be stamina -- but that will ease with time.

I do have a third-hand sense that they have all types of people, especially during the package season.

I did it. I did it when I was in college. There aren't many people your age who do it BUT.... I assuem the job is a little different and there are a few different jobs. There are two main jobs: getting boxes off of conveyor belts, and "small sort" Small sort was the girls' job usually but you basically did what it says, sorted the small packages. I had a head for numbers and was strong so I did regular packages. This means you stand by a conveyor bet and grab boxes and look at the zip code and put it on one of 10-12 other conveyor belts behind you. A lot of lifting thigns from waist height and twisting. Heaviest stuff is 70 lbs but this could be en entire truck of boxes of paper. Usually if you get a truck like this, the other package people will help you out. Sometimes they don't because they are assholes and you are a butch female and they like to watch you sweat.

I quit after my boss called me a "skirt" but other than the stupid sexist atmosphere it was a great job. I got in shape, enjoyed myself and it was union so not only paid well but they were totally decent about breaks and all the other money/hours stuff.

Cool.
On the cardio side, I think I'm in fairly good shape. I do a 3-mile treadmill run 2 or 3 times a week. Well...closer to 2 time a week ;-)
If I get lucky and land the job, any suggestions on shoes?

I quit after my boss called me a "skirt"

Criminey, when was this, the 1940s? How rude.

any suggestions on shoes?

Brown.

I worked there the summer after my junior year of college, so 1989. I had a nice exit interview with UPS where I basically said that while working conditions were mostly decent, they had a real asshole problem. I'd just wear sneakers or workboots made for standing (no metal toes!) with good soles, it's flat surfaces and inside. I don't think you'll have a cardio problem, mostly getting good at lifting and turning back-wise.

Watch for falling and flying boxes. I am not trying to be flippant. Both my friends who worked for UPS were hurt this way (both female, if that means anything).
The first friend had a 20 box thrown at her as she was turning, and she didn't see it in time. She said it hurt like hell but luckily she didn't break her arm -- just an awful 12" bruise that took forever to go away. The other friend had a box dropped on her foot (I'm fuzzy on the details). But luckily her foot wasn't broken and UPS paid for the doctor's visit (after tons of paperwork).

More anecdata: My (healthy, strapping) 24 year old brother-in-law only lasted a few weeks before the arch of his foot collapsed.

My father worked as a mechanic for UPS for many, many years. He says most of the package handlers burn out pretty quickly.