Uphill struggle

Heading home tonight feeling tired and a bit sore. My age is catching up with me and, even after a rare good night of sleep, I'm still knackered by the time I leave the office. I've been getting the train to work, instead of driving, for the last couple of weeks and the walk between the station and the office is meant to be doing me some good. So why does the walk back to the station seem to be getting harder, rather than easier, and why is my waistline not even slightly responding to my increase in exercise?