Premium probably helped, but being near full would have been more effective to keep out moisture as Fred said. My Ventura sat for 18 years before I got it, and much the same scenario, tank was nearly full of nasty crap that barely burned with a flame held to it, but once it was drained and flushed out it was very clean inside.
I rebuilt the carburetor and replaced the fuel pump, reverse flushed out the fuel line, and I also changed the fuel filter every month for the first year or so. Been no problems since except I had to change the pump again a couple years ago.