Allow users to earn badges for completing a particular number of kilometers of cycling, running, or other sports. For example, once you log your first 1000 km on Strava, you receive a Bronze cycling badge. Then silver for 5k km, gold for 20k, diamond for 50k, etc. Badges can be showcased on the profile page or even on the profile picture.
Another way of doing this would be to give a certain number of experience points for each logged kilometer or minute of cycling, running, or other sports. That would allow unifying all sports activities into a single "XP" metric.