Yep!
Endurance, endurance and endurance.
To the OP, you just started driving? What was your last job inside, if any, within building? Driving is very hands on with a lot of endurance. Picture playing a professional sport ok. Playing the sport is the easy part, but the training for it is the hard part. If you were not walking as much or lifting as much in your pervious job, then this will be common problem until your body adjust to the work load.
I like the comments made here, most of these drivers here are always looking to make their job easier. That is pushing past the physical aspects by doing other physical activites on the side. I know one driver here that is a marathon runner on his vacations, and trains often on the weekends. Hes in his upper 50's and rides a bike to work and rocks a body like a 20 something. One of the best drivers in the building. Its almost like he uses the driving job as more training. And is always smiling no matter the load amount or quality.