I can tell you two things.
I remember a few times when I was a kid, playing soccer, they would send me off to run laps. I remember one occasion, I think I was almost crying I was so tired... and suddenly this dreamy feeling came over me, where I felt wonderful and I could keep running, and it was effortless. I only remember that feeling happening once.
During the summer, when I had time off from work, I was getting some really nice workouts in, and for some reason, I had this workout that left me feeling really really upbeat and happy. It was different from the usual "I feel good ´cause I moved my butt" feeling. But it wasn't the same feeling as when I was a kid.
Were those examples of the mysterious "runners high?" I don't know.
What I do know, is that while I´m exercising I consciously focus on how my body feels, to observe and identify the sensations I'm experiencing. Usually it feels pretty good. I also am very careful not to do too much.
If you are just starting out, just do what feels good and really focus on that. Reflect after your workouts - Did that feel good? How would you feel if you hadn't just exercised? It has taken me a long time to learn to enjoy exercise, but I think I'm getting there... !