How should I model this data?
What you've modelled is a snapshot of the user's total progress.

I think you might want to track the user progress a little differently.  Instead of letting them enter their distance covered directly, why not track their way-points?  If you make a kind of audit trail of where they went (lat/lon) and when (date) then you can easily calculate their distance covered using Vincenty or another numerical method.

By tracking the progress on a map (i.e. by lat/lon) by date you can have more interesting reporting of the data.  For example, what if two "contestants" want to race but can't start in the same place at the same time?  How do you compare their progress?