It would be nice to have stepped challenges that have different levels for people of different abilities to shoot for. You would then get a badge that would indicate which level you had manage to complete.
For example, you have to be very serious about your cycling to average 40km / day for a month to collect the cycling distance challenge. If it had steps at e.g. 50km, 100km, 250km, 500km, 750km, 1250km it would provide a challenge for everyone, whatever their level.
I realise that you could just have a series of separate challenges (in the way that running has 5k, 10k, half marathon) but this seems clunky, and doesn't provide the automatic incentive once you complete one level to push on for the next one.