The half marathon filled up within a week last year for this year's race. The marathon takes a little longer.
Last year, they had people who wanted to run the half on a waiting list, but they ended up opening up 2,000 more spots because of the overwhelming response.
And yes, you can walk, but you must maintain a 16 minute mile. Though that's iffy because I volunteered at last year's event and I was at mile 16. They were just rounding people up around there who weren't at a certain point. Since that is after the half-marathon end, I'm pretty sure if you go a tad slower, you'll be all right.