Almost exactly what you described - turned out to be a bad battery (which was pretty old anyway).
I stopped for a minute to talk to someone and shut her off. Could barely start her after a few tries. Thinking back, this condition was worsening in time. and, at some point I couldn't start her at all. I would charge the battery overnight, and she would fire up, but a week later the same thing would happen.
With the new battery for many months now, and no problem.
I think the way the battery goes bad is in the area of cranking power.