I have no doubt that you are physically conditioned for your job.
To put this argument into perspective, imagine that you spent your day driving, doing maybe 8 stops per hour, and delivered mostly "flats". Your physical condition would be markedly different from what it is now.
Now, take an Express Courier that has been doing the above scenario for over 5 years, then throw them onto a route where they are required to get off 12+ stops per hour, dealing with pieces that weigh on average 40-60 pounds - they aren't physically conditioned to that sort of activity, and they would end up hurting themselves after about a month of doing that.
I had this argument with topped out Express Couriers when I was in - they knew they had the "easy" routes - and weren't going to risk what they had (by signing a rep card), to possibly get a few bucks an hour more (they wouldn't benefit NEARLY as much as those at the bottom would if Express organized). They knew that they had to keep their easy route, or they'd be practically signing themselves out of a job.
That was part of the very real threat Express made (and makes) to its older Couriers - if they start agitating for change, Express will start reassigning routes and the older Couriers will be out of a job within a few months (they'll hurt themselves on a heavy route, end up not being able to work for over 90 days and then be out of a job). Oh yes, they could come back if they took more than 90 days to recover, but they'd be left taking the worst route in the station - which they knew they couldn't handle. Those over 45 know not to agitate too much, they'd end up agitating themselves out of a job.
All "industrial athletes" know that a large part of the job is conditioning. Many Express Couriers aren't conditioned to handle the heavy routes - they are used to the light routes. They do literally live in fear of hurting themselves, being out for more than 90 days (losing "their" route), then end up being given the worst route in the station when they return. For employees who are out for more than 90 days, seniority means NOTHING when it comes to coming back and getting a route - they are given what is available.