This page has information on hill/fell running.  Most weekends there are fell races throughout Wales.  Information on those can be found here: 

Welsh Fell Runners Association (WFRA)

Another useful source of information on some local fell races is:

Fell Race.com

Some fell races can be quite long and tough and involve some navigation but many are shorter, marked with less climb - well within the reach of a reasonably fit 'off-roader'.

 
  
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