I have been back to carrying my Brompton around the tube system this week and I noticed how much easier it is to carry up and down stairs, even when I am carrying a load of stuff in my Timbuk2.
I could always carry the Brompton but it was an real effort to do so at the start. My knees and arms would hurt and I would need to pause to catch my breath. I did wonder if I should have brought a lighter frame as I thought I would never get used to it. But is really was just a matter gradually building my strength and stamina, which I have. (If I was in my 20s this would be a breeze.)
Now I can carry the bike with one hand, no problem.