Well, when I worked as a mail man I did it with school so I was a surrogate mail man. If a regular guy was sick I'd get his route that day, so I'd start off later in the day, after school, and since I didn't know the route and names on each mailbox by heart it would take longer, especially for apartment buildings. For some of the larger blocks with maybe 20+ apartments I could be up to 10-15 minutes just sorting through the stack of mail. On rare occasions I'd have the same route maybe for an entire week at the end of which the time would be reduced quite a bit as I learned the route and names better.
But you'd be amazed at the stuff you learn doing it, like old people being super pissed off that their mail didn't arrive at exactly the same time it does with the regular mail man, or people who are pissed off that you rang the doorbell to see who lives there when they can't be bothered to write their name on the damn box, or my favourite (when living in northern Europe during the winter) not ever bothering to shovel/de-ice your steps making that 6 step trek to the door almost as dangerous as climbing the north face of Everest unless you remembered to bring your studded galoshes. Combine slippery steps with absolutely no lighting what so ever and it can get pretty tough.
Oh, and the weather. Strange how, when you're a surrogate mail man, the worse the weather is the more likely someone is going to call in sick and you'll get a run! I've delivered mail in some pretty shitty weather, but only once did I give up. It was mid winter with snow and ice everywhere, but the day started fantastic, clear skies and hardly any wind. When I left for work after school, little did I know that the weather service had issued a notice for a rare, super quick storm brewup. I got an extra heavy cart that day, weighing in at about 60kg (more than I weighed back then!). A third of the way through the run the storm broke but I kept on. It wasn't until it took me over 10 minutes to cross a parking lot (maybe 50 meters) to get to an apartment building that I thought, fuck this, I'm calling in for a pick up. My boss was not very impressed, but when the van arrived the driver and I had real trouble getting the cart into the back, but once at the office he told my boss just how fucked up the weather was there.
Anyway, I'm glad I'm not working for the mail any more, I do have some respect for those who deliver the mail, unless they're complete fuckwits. I always make sure my name is on the box, I keep the light on the porch during winter so they can see, and I clean my steps so they don't slip and break their necks. But when they either put my mail in someone else's box, or put someone else's mail in mine then so help me god I call up their boss and complain!