IMO, a 5k and maybe even a 10k is too short to "feel easy" -- for me anyway. They are run too close to the anaerobic zone. More often than not, a good 5k time is run while feeling like being on the verge of blowing up from mile 1 on.

I've had that "feel easy" feeling in 15k -> half-marathon races. They are short enough that you feel like you are running fast, but you are going slow enough to enjoy it.

In longer races I can have that same feeling during the middle of the race. But it is eventually replaced by the aches and pains from the pounding endured by the length of the run.

ps "Runner's high" is overblown and overrated. If you feel that good, run harder. :-)
 

Dissociation and "runners high" are two very different things. Most of my PR's have felt a lot more like Boris', particularly in the early going. I agree that it takes at least 15 minutes to get into this state, so it's not too likely in a 5K.
 

I have only one PR that has stuck in my mind over the years, a 3 mile qualification run in the service. I'd tried to break 18 minutes all through high school and could never shave the last 10-20 seconds off. The final qualification I took in boot camp I ran like I was two feet above the ground. It was absolutely effortless. I ended up beating one of my drill instructors for the first time that day. I used him to pace me and then passed at the finil sprint. After the race he congratulated me and related that he had misapplied some icy hot which had got up in his shorts. He didn't realize his error until mile one and was pressing the pace to end his suffering sooner. That was over 25 years ago and I can still remember how effortlessly I ran. It reminded me of hitting the sweet spot on a golf club. Ting! Nice and easy.