Being an 'old bloke' has its advantages, although I wouldn't exactly call you old. One of them is that people don't expect as much of you, and another is that you should no longer feel compelled to show off your strength.

And as Jackie said - the female manager probably has a knack with those packs - and if she doesn't, depending on how long she's had that job, she probably has developed 'laminate floor pack carrying muscles'.