To me the runner's high monent kicks in at some point in the training when my body has suddenly "got it" and the training that was previously demanding and that I really had to consciously work at now feels like second nature to my body. I feel my body to be running like a well-oiled machine, my breathing is rhythmic and steady and as you say, I feel I could just keep going for miles and miles!
I never had goosebumps with it when running, but I get that feeling when I am doing the final challenge part of a Cathe cardio routine: the sudden rush and I can go all out for the final blast, a sudden second wind of energy.
Be careful though: even though you suddenly feel that you can double your mileage in a single day, running magazines do not recommend that you do this. They advise that we not increase the training by more than 10% at each stage of training. So even though you suddenly want to do an extra 3 miles, just do the 1 extra, then cool down. This is because your joints need to be slowly acclimated to the extra demands and impact. Doing too much extra too soon could lead to an injury that sets back your training and p*sses you off mightily!!!!!
Clare