Medals are earned by completing a session for an exercise type 8 times. For instance, the Run the World type has 10 races, but only 8 are needed to earn the medal. Conversely, the Jump Rope activity only has 4 sessions and so one could complete all sessions and still need to complete 4 more.
You would think it'd be easy to just complete the shortest session 8 times, but you'll want to complete all sessions if you are going for the following achievements:
Furthermore, since there are badges earned for completing all sessions in each activity doing this will also get you well on your way to earning this additional achievement:
And lastly, if you want to continue earning medals you will ultimately get this:
When I earned this achievement I had medals in most of the 1 player activity classes. They are the shortest sessions and a few of them don't even have 8 sessions so you can complete all sessions (earning you the badge) and then just replay the easiest/shortest session.