That was my approach in the challenge: just to do my training plan. I had a self imposed rule that any extra runs (eg, a 31min run for extra credit) had to be on top of my real plan
... And not on an off day. Still need rest.

As they said, since you are right on track with the challenge no reason not to sign up. If your runs are on strava then you can just sync and everything will populate overnfromnthe beginning.