I ran it in 21:39 (unofficially - that's per my Garmin), which is a time I am quite proud of.
The biggest deal, though, is what I learned (remembered, actually) about racing.
I forgot about the adrenaline rush you get when you start.
I forgot about the crowding you get at the start. I spent the first mile weaving in and out of people.
I forgot about that last mile, when you have to really push and fight through pain and fatigue.
I forgot about how good it feels to be done, and how great you feel afterwards.
The 5k is a much different race than a marathon, of course, but I got some great experience in race day mechanics. This 5k was about 30,000 people (even though only about 2,000-3,000 competitive runners), which is about the same size as the marathon will be. That will help me control pre-race jitters (I had plenty today and last night).