One already existing example of this would be in fund raising events where a person pledges how much money they will pay for every mile a person walks.
Yes, there are a number of opportunities for achievements for such a real-life event. The organization hosting the charity walk could award achievements for entering the walk, walking 1 mile, 5 miles, 10 miles, etc., award special single-award achievements to the person that walked the farthest, the second farthest, third farthest, etc., similar awards based on funds raised, etc. Individual participants could create and award achievements for sponsoring them, being their biggest donor, etc.