If you signed up with Love To Ride (for FREE), you can easily set up a goal to track your 100miles of riding for the Pedal The Prairie Challenge #PTPC. If you are tracking your workouts with Strava or MapMyFitness, you can also link those apps/workouts with Love To Ride. More info here.
Love to Ride: allows you set a goal for a specific time range and distance. This can be done from from your account. Click on the Goals tab and set up the goal as shown below. Currently, there is not an option to do it in the group.
You can not back date. You can only start from the current date to August 31, 2021.