You did two marathons in the last ~six weeks. Many people take 2-4 weeks off after a marathon, and theirs don't usually include 60 miles of biking and a half dozen hours in a canoe, all on four hours of sleep. You worked hard and crushed in those races and you are entitled to feel lousy for a little while!
As far as Crossfit and 5ks...the
average male Crossfitter attending the 2010 games couldn't beat a
16 year old at my town's little 70 person 5k. It undoubtedly has benefits, but endurance running is not one of them. Hell, I could beat the
top ranked guy in a 5k.
Usual caveats apply, I don't race as hard or fast as you (I coasted through the Rage, not having to to keep up with Tony, Phil or Mike).