I noticed this first in museums, and now it's coming up in birdwatching. Walking slowly for a couple of hours makes me exhausted.

I'm not an athlete but I'm quite fit. I don't own a car, so I walk (fast), Xootr or cycle everywhere I go. I carry home virtually everything I buy.

On my last group birdwatching venture, someone actually asked if I was okay (I was, but I was fading).

Anyone have ideas? Is this some metabolic state, or is it just me? I am actually avoiding birding with other people (it's city birding, so I can Xootr when I'm alone).

Pam