A possible solution could be, if a "hall of fame for friends" would exist. For example some participants or friends agree on certain values (for plausibility) and they get a private hall of fame. So the competition and therefore the motivation of many of them will be certainly higher, than if they never have a chance to be part of the overall hall of fame.
A special kind of private hall of fame could be a combination of two or more sports, so the kcal gets cumulated, i.e. for biking and jogging. Also great would be to define a time frame within the competition is going on.
A member of Run.GPS should be allowed to take part in as many private hall of fames as he/she likes.