- start challenge
- goal=1000km
- startdate=now
- enddate=now+1y
- store in shared pref
- show current distance + goal
- show "calendar" view (one box for each day, green if workout was done, (now - startdate) boxes are shown), horizontal scrollview
- add workouts
- manual
- via map recording
- share current screen
- group challenge (invite to challenge)