The endorphins come later. Much. Much. Much. Later.
There's this thing you hear runners talking about called the runner's high. I always considered it a mythical thing. Like heaven, it's a place where you no longer hurt, breaths come easy, the cramp in your side disappears and you think you can go on forever. When I was running on a treadmill, I could go for hours and I mistakenly thought this was in fact the 'runner's high'. In reality, I was in fact doing it wrong. Turns out you're not supposed to have it on the lowest setting. This simulates a slight decline, meaning you're in fact running downhill, and we all can do that forever. (Only a slight hyperbole) The professionals and die hards say you're supposed to put the treadmill on a one to three percent incline. So, oops.
Outdoors is a completely different matter. When I first started running outside, I noticed it was a lot tougher on ever part of my body. Meaning I worked much harder. Suddenly those hour long running sessions were a thing of the past. Five kilometres was where I drew the line. Except, I've been getting stronger. Not skinnier. No, my thighs are the same hammy girth, but I can walk farther. Jog longer. Move faster. And it isn't so hard anymore.
So, I decided to run down to the lake. It's only five kilometres there, but when I arrived, I decided to run back. Ten kilometres. This was the farthest I've ran outside on the rugged terrain. Sure, I've done fifteen and twenty on my adventuring, but never running the whole thing. Truthfully, I walk a lot. But I did run the entire way there and back. Not walking once. I am proud, even though it is a drop in the bucket for those nutcases who actually run half marathons, marathons and ultra marathons. And about seven kilometres in, I hit the runner's high. My stride was strong, my posture perfect, breathing easy, and I felt good. Strong. Healthy. Unstoppable. Just between the two of us, I actually contemplated adding more distance! Of course, this was insanity and I snapped out of it and went straight home and into the bath.
As it turns out, the runner's high does exist. It just doesn't happen for awhile. You have to put time and distance in before you can push back the 'Gee, this is terrible' part of your run. Of course, this is all coming from a woman who's thighs get separation anxiety when in the cobbler pose because they aren't used to being apart. So, take it for what it's worth.