Mysql – How to model this relationship

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?

i will recapitulate your description:

• A user can create lists.
• A list tackles some tasks.
• Users can join some lists.
• A user can send an invitation to some users and propose to join some lists

Based on this description I would propose the following ERD

From this I would deduce the following relations

user('user_id')
list('list_id',user_id(creates))
joins('list_id','user_id')
task('list_id','task_id')
invitation('invitation_id',user_id(sends))
receives('invitation_id','user_id')
proposes('invitation_id','list_id')

Primary keys are between apostrophes. Foreign keys can be deduced by the names if the fields (so user_id in lists references user_id in user) The word in parantheses after a foreign_key describes from which relation in the ERD this foreign key is derrived.

I did not care abou the names of the id-columns. Perhaps an id is a surogate key (an artificial key) or some attribue of the entity. also i did not add additional attributes which can be easily added. The characterisation o task as a weak entity is rather arbitrary.