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What miles can I count? Here's the deal... You should only count miles when you go out for a ride for the purpose of riding or getting miles in. You can do this on the trainer, roads, trails, wherever. Bike communting counts, too. Riding your cruiser to the corner store for a six pack, however, doesn't.
Can I sign up after the race starts? Yep, but if you sign up after the race starts, your miles start counting on the day you sign up - not back when the race started. Everyone has until October 4, 11:59 PM ET to finish.
Can I count miles from another event/race? Sure thing. As long as you ride the miles during the event dates (after you've signed up), you're good to go.