Well, I feel even dumber now.
I got on the treadmill with the incline at 0% and guess what? I had NO problem at all. No heavy breathing. No tired legs. No sense that I had somehow gotten out of shape overnight.
What a maroon, as Bugs Bunny used to say.
I was able to stretch the session out to 4 1/2 miles which was what I was trying to do last week before I accidentaly set the incline to 4%.
ORN: 4.5 miles. 4 miles, R5/W1. 1/2 mile walking to cool down.