runner's high
noun
: a feeling of euphoria that is experienced by some individuals engaged in strenuous running and that is held to be associated with a release of endorphins by the brain
Example Sentences
Recent Examples on the Web While virtual events filled the time between 2019 and now, a true runner's high isn't complete without an official finish line (or Finish Swine in this case) along with the accompanying medal. 
Scott Springer, The Enquirer, 1 Nov. 2021 One thing that comes close to rivaling a runner's high (or second wind) is the satisfaction of snagging a really good deal on brand new workout gear. Karina Hoshikawa, refinery29.com, 25 June 2020
Scott Springer, The Enquirer, 1 Nov. 2021 One thing that comes close to rivaling a runner's high (or second wind) is the satisfaction of snagging a really good deal on brand new workout gear. Karina Hoshikawa, refinery29.com, 25 June 2020 Word History
First Known Use
1975, in the meaning defined above
Medical Definition
runner's high
noun
run·ner's high ˈrən-ərz- 
: a feeling of euphoria that is experienced by some individuals engaged in strenuous running and that is held to be associated with the release of endorphins by the brain