“I think I’m reasonably fit. I do a lot of walking (dozens of miles a every week), but I get so out of breath when I climb the stairs/run for a bus/play with the kids. What is wrong with me?”
I recognise it. Every single week someone comes up to me and asks me some version of this question, so here is the answer.
It is all to do with the Intensity/Duration Curve. First of all, here are a couple of definitions: