Welcome to the Community @MGollery. You have to link both accounts. I recommend going to your MapMyRun account and search for an option to Link it with your Fitbit account. If you have any inconvenience trying to do this, you can contact MapMyRun customer service directly.
Keep the stepping up! 
Alejandra | Community Moderator, FitbitIf you like something I recommended, I encourage you to mark that reply as "Best Answer". 🙂