Don't worry - 420 is a perfectly fine baseline score. Remember, you're just beginning your prep, so your score is bound to improve.
As for extra work, I think you'll find that there's more than enough work in each of the day's activities.
If, after completing a day's learning activities, you find you want more, you can always spend some time on the BTG forums answering/asking questions related to that day's activities.

I hope that helps.

Cheers,
Brent