Neurological explanation for differences in what motivates runners? I have noticed differences between how my friends and I are motivated with regards to exercise, and would like to better understand what is influencing this difference psychologically.  

Personally, I am indifferent to running events.  I have run for its own sake on my own, both at novice and intermediate levels; training myself until I could run 5k or 10k regularly.  I once did a half marathon distance on my own simply as I was curious whether I could do it.  I feel absolutely no motivation to run as part of a group, nor do I feel any motivation to run for the reward of getting a time or rank or medal or what have you.  

However, I recognise that this mindset is unusual when compared to my friends.  One friend started running with a "couch to 5k" group, another is keen to train for a half marathon event, another is a semi-professional athlete, etc.  The point is that most people I know who run seem to be motivated differently.  

What explains the difference between these types of motivation?  This is not to ask what drives motivation generally, but specifically what is the neurological and thus psychological root cause for individuals being motivated by different sorts of things, in this case about why people run as a form of exercise.
Your question is really broad and can encompass libraries. However, to add my 2-cents worth regarding 


  specifically what is the [...] psychological root cause for individuals being motivated by different sorts of things...


I would say that a lot of this has to do with external factors, and specifically external sources promoting exercise to be 'healthy', including peer pressure and multimedia.

To pin this global effect on our way of thinking down to  


  [the] specific[...] neurological root


is not really feasible in a single answer in theSE framework, as it is just way too complex to grasp on a single web page in a Q&amp;A forum I'm afraid. But the between-subject differences can, at least partly, be explained by intersubject environmental differences.  

Reference
- Brody &amp; Inoue, Issues Innov Nursing Practice, 218-27