Kurt ran 3:38-3:39 after college, in addition to a smattering of other strong performances.
He was - and is - a massive talent. This is a fellow who ran right around 13:30 for 5000m as a true freshman at Notre Dame, for god's sake. Like many other great lights before him, though, he was felled by the injury bug.
Observe his running style in the video: he's got a fairly loping stride combined with an extreme forefoot strike. From what I gather, this has contributed to some nasty, chronic achilles issues over the years.
Bottom line: yes, Kurt could have kept up with Molly for the full workout, but he probably wouldn't have been able to run again for several weeks thereafter. He recognizes that he can better support her if he stays reasonably healthy.
He's a good man and a supportive husband.