Well, it looks like I have some work to do. Rode all of 7 miles, almost half on sidewalk, the other half on very easy dirt -- and I was beat! I thought that managing a 280 lb. dirt bike for hours of riding put me in at least decent shape, but noooooo.
Maybe I would have fared better if the temp wasn't 103.
In any case, I appreciate all the replies. It would have been easy to tell myself I need the latest and greatest bicycle after seeing what's out there. But seeing how I'm the weakest link the riding chain for now, I need to spend weeks and weeks and weeks riding and see if I can't get myself into decent shape.