Chris,
I've been having good success with Lily lately using a gently sloped portion of the yard to teach her balance and get rid of the training wheels. She's got the blue version of that bike and I think one size larger. Andrew hasn't exactly taken to his balance bike either this summer, which saddens me. I'm guessing next year he'll be on something with pedals.
And if you can manage to pace yourself on the century you'll be ok. I had just enough time to recover from a stupendiously lazy July and hope to be ok next weekend on my century. I'll still be pretty watchful to eat and drink more than I think I need to avoid bonk/cramp issues.