Good point, Millsy. Since the Equality Act 2010, it would seem that you can't legally exclude people on the basis of gender (or any of the protected characteristics).
Having had a little read of EHRC guidance, a running club which accepts anyone who applies for membership (eg. there is no nomination or voting process by current members for acceptance) is treated as a business.
Businesses cannot legally choose to supply their goods/services only to one gender or the other.
Edited: 02/01/2013 at 16:54