I ran up against my physical limitations yesterday when I tried jogging for a bit instead of just walking. I could do the running without tiring, but I quickly hit the edge when I started breathing heavily. Or, at least, trying to breath heavily. There just isn't much depth to my wind anymore - I couldn't get that deep swing-intake of air necessary to keep going for more than three hundred yards or so.
I'm not coughing or anything, or wheezing, or suffering in the usual asthmatic ways, but there's just no lung capacity to spare for anything more than a short sprint.