Simply put if you got six thousand miles with some night driving the alternator must have been driven. Set up the way you suggest the alternator light would have not gone out after starting if the light was working. If you had a single pulley on the alternator and a double pulley on the waterpump or fan. The wrong alternator pulley was present. They did not have a belt that would be non adjustable between the crank and the water pump only as well did they?

Also For the single pulley to go bad it must have been driven. First measure the belt you removed. It should be between 39 and 40 inches. I no longer will be using the aftermarket car belts for the twin pulley set up. Instead using the 39 inch utility belts available at hardware stores. They install a little harder but not much and give you a fuller range of adjustment. Or that is my experience.

They just seem far better and do not initially seem to stretch in use. The normal car belts seem to be seriously lacking at a much greater cost. My last auto aftermarket belts were continental and junk in comparison.