Source: http://www.thefitbits.com/2016/01/when-only-run-will-do.html
It doesn’t happen the same with anything else.
When I’m swimming, I don’t suddenly feel amazing half way through. When I’m boxing I’m the most knackered I’ve ever been, but the rush doesn’t come until I’m done.
When I’m cycling it kind of happens, but it has to be a long challenging ride on some proper beautiful roads or trails.
With running, it’s different.
It takes me about 20minutes to really warm up but once I do and if all goes to plan, there comes a point when everything just works. 
Do you know that feeling?
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When your body is like an engine and it just keeps turning.
You look down at your watch and see that your ‘comfy’ pace is faster than usual.
Your legs and breathing are in perfect sync and your mind can wander effortlessly.
You soak up the endorphins and smile to passersby, stopping to take photo…