Running marathons is not a good comparison for bicycle touring. Marathon runners often take in no calories at all during the run, and if they do, it's almost certainly liquid (e.g., Gatorade). As a cycle tourist, you will have to take in calories as you ride or you won't make it. And your recovery will rely on you being able to take in more calories after you stop.
I think you should ask your doctor how many calories it is possible for your body to absorb over a given period of time if you have the surgery. My guess is that this is limited (which seems to be the point of the surgery). I seriously question whether you would be able to successfully ride six straight hours after the surgery. But I would certainly think you would be able to do the TransAm anyway. The question is only how long it would take you. Maybe you could alternate an hour of riding with an hour of resting.
I agree with your objective to find another long-distance bicycle tourist who has had this surgery. You may get lucky here (or over at CGOAB) and find one to give you advice. If not, however, your doctor can hopefully guide you--using better equivalents than marathons. Specifically, you should ask your doctor what strategy will allow you to engage in vigorous exercise for six hours a day, day after day for months. If this is not possible, then ask how many hours of vigorous exercise you will be able to do a day, day after day for months. Even if it's only three hours a day, you can still get across the country. It'll just take longer. But hey, you'll be retired. Practically, you'll have at least from May through September. That's five months. Even at three hours a day and 9 MPH, you'll make it across.