It's definitely something that happens to me now and then, on easy paced runs, after I'm warmed up.  There are days where everything is just clicking along nicely and I start feeling so happy just to be out there with (insert whatever birdies, flowers, dudes in boats playing the harmonica, or whatever happens to be around).  And yes, I will run an extra little bit pretty frequently when that happens, just because I don't want to stop.  My DH makes fun of me because I do get all blissed out, and sometimes that lasts for a bit after I get home.

Or I will take him on a walk past this really cool thing that I saw when I was running, only it's not all that cool once we go back and I don't have the endorphins going.

I also definitely experienced it during my 10-mile race.  I need to have a race like that again.

I can't shed any light per se except that I know over on the old RWBF somebody clarified that it wasn't endorohins really but something else and that it wasn't as common as legend suggests.

For me, it goes something like this.  I have 2 kinds of "feel good" feelings I get from or while running.

1.)  After a good run, the feeling of accomplishment brings instant gratification and allows reflection enough to enjoy til my next run.

2.)  I sometimes get a feel-good feeling when I'm running and it feels effortless whilst also running a bit faster than usual.  This doesn't happen as often but it's a great feeling.  I feel like Ryan Hall looks when he's going along at his race pace, even though I'm nowhere close to such a pace.  Some people describe it as that *floating* feeling which I can agree with, too.

Certainly pain, sweat and exhaustion take precedence over any "good" feelings. 

It happens to me quite rarely, but it does happen.  A couple weeks ago I was doing some tempo  intervals.  They were slightly easier/faster than I expected, and as I was doing the last repeat I definitely had a bit of a high, felt like I could run forever, just felt happy/strong/confident all at the same time.  I actually forgot that I had somewhere to be (work) and think I would have done an extra few miles if I wasn't going to be late.

Relevant (on the 'flow state')

(Describes my experience almost to a T... I am sure you could argue about whether 'runner's high' and 'flow state' are or are not the same thing.  Psychology Today seems to think that runner's high may be a form of 'flow state')

Sometimes while running I start daydreaming about when I can lie down and take a nap.

Why isn't there an every now and then? I know I've gotten that "high" during runs damn it, not often but when I do they are amazing.

I picked all the time, but this would be more accurate. Usually happens about 40 minutes in, where I feel like life is just grand and I could keep moving my legs forever.

I get a sorta-similar thing going on when pool running sometimes too, happens in the same timeframe but to a lesser degree because pool running is just not as nice as dry-land running and I don't actually want to keep doing it forever.

Mainly during my long runs, or runs longer than 6 miles. Especially in colder weather, but not too cold.

Also, in many of my marathons, I've experienced the following: First 8 miles is like "Urgh... I don't think I like this. Why does it feel so hard?" (could it be because you're going too fast, you moron?). Then, from mile 8 to 13, it's like "Okay, the pace feels better, but mentally, I don't have it today. OMG, I think I'm going to have a panic attack. I'm a sham, I'm not a real runner!". Then, between miles 13 and 21, it's my almost runner's high: "What?!!!! I'm at mile 21 ALREADY?!!! I did NOT see those 8 miles at all, did I really run them?". Then it's the last 5.2 miles, the opposite of runner's high. More like runner's low.

LMAO.....LOL, " I think I'm going to have a panic attack.  I'm a sham.  I'm not a real runner!"  LOLOL  Omg, I swear I start every race like that.

I've heard it described as Meghan has, as a flow state.   I haven't, however, ever experienced it as far as I know, though I do often have feelings of joy and peace while running.  I think it's simply because I like running, but I'd never describe it as an altered state.