Oh man it is 70 degrees today and gorgeous, and I totally want to go for a run! Not 12 miles, but I was considering 4ish. I've only ever sprinted on turf or grass in my vibrams, so I don't know how I'd feel running 4 miles on pavement though.
I think every once in a while is probably fine, if you really want to do it, then do it! As long as we don't get into the run more to eat more to run more cycle (at least that was me before), then you should let yourself have fun I'm going to be hiking for hours and hours next Friday, and I can't wait.