The walking part of it was one of the things that jumped out at me. 15 miles is a hike. To do that every day on concrete would tear you up.
Since you work at such a place, what is the feasibility of motorized "scooter carts", that you could ride on, perhaps standing up, as you go from location to location?
15 miles is a hike, but it wont tear you up after you do it for a weeek or 2. It only feels like a lot because many office drones expecation of amount of walking a day is anchored at "sit at desk all day." Appalachian Trail hikers start at 8-10 miles a day and work up to around 15 miles a day on a dirt trail over mountains while carrying 1/3 of their body weight! And they do that every single day.
15 miles a day is no big deal, to say nothing of 7 miles a day.
We use powered pallet jacks (http://en.wikipedia.org/wiki/Pallet_jack). The catch with those is that a lot of items are close enough together in the warehouse that most guys end up walking alongside their jack. Still, it beats manually dragging two pallets' worth of product on what could end up being a two mile trip per order.
Since you work at such a place, what is the feasibility of motorized "scooter carts", that you could ride on, perhaps standing up, as you go from location to location?