I finally heard from the man himself with a brief and not so detailed message. He arrived at his first day's destination last night around 9:00 pm (MST) feeling tired but otherwise great! There was a hint of surprise in his voice at how good he was feeling (though it was no surprise to me - the man's been training his tail off!). After some pizza (food of champions!) and a shower, he hit the hay to prepare for his 3:30 am (MST) departure the next day.
Here are the stats he gave: in the end, he ended up riding the 253.1 miles of the day at an average of about 16.4 miles per hour (pretty great!). He did his own personal best on the 400K at 17.53 mph. I don't know about y'all, but it sounds like this is turning out to be a very successful ride so far. I hit you with more details as they come (and hopefully pictures will follow once Stephen returns to DC).