First of all is that you must have at least sleep well and eat well. This job is more on moving, lifting and sorting mails. Probably, you have to prepare yourself everyday because this job is more on physical tasks.
First thing i would do is check out the truck i was using that, then go back inside and sort letters . then i would organize whatever packages i had i deliver and load them into the truck yo make it easy grab. then i would load the truck with all the mail for the day. then i would go and deliver it