It just would seem unfair if the top 3 people end up in the same team since they are ahead by so much but I think there will always be a chance of that happening.
I'm just going to throw idea out there.
Make the top 2 or top 3 runners after this first challenge be captain and people could pick which runner leader group they want to join and go from there. First to 1,000 miles, most miles in a month, etc...
or
Pick two rivals (with 50/50 in followers) and make teams for each and let people side with whichever they like better. ie: Team Marvel vs. Team DC, Team Iphone vs. Team Android, Team Pacman vs. Team Mayweather, etc...