Overselling a bit there. I've never felt anything I'd qualify as euphoria from exercising. The closest is the runners high which is, for me at least, really not that high.
I know the runners' high too, the runners' crave also. And long walks (forest + small hills) does something a bit different because it's a low intensity effort. Yet after 40+min you can go from depressed to a thin and constant "life is beautiful" background emotional state. That's all.
All in all the takeaway point is: doing something stupidly simple for long can change your mood quite surprisingly.
May I add that when my cardio vascular system was fubared, this kind of walks (moderate uphill) was the only thing that could interact with my body the right way. Being almost bed ridden, but an ex athlete I tried running, jogging, biking, indoor biking, taichi, stepping. They were either too demanding and injuring (for my very weak self) or of low efficacy. Yet walking had the right rhythm, full body muscle use, right intensity to circulate blood everywhere. You can guess how good it is to finally feel warm blood reaching your fingers or toes. It also helps your breathing, lungs ..