Hi Chad, thank you for sharing your experience so openly, I hope it might encourage more people from cycling try running as well. Especially those who might be time limited as running so much less time consuming.
I think (and this is just another anonymous letsrun hobbyjogger) that the years that you have spent cycling at extremely high level and your current running results clearly show that your cardiovascular engine is more than capable of producing OTQ type of result. When you add that you have good size/height in 5'10 and 120-130pounds, ie not 6'+ person who might see some diminishing results when the distance increases, the only things that are limiting you is your economy (which is already reasonably good as evidenced with 68min half) and your muscle/tendon/bone ability to withstand both the training for marathon and racing the marathon. As the marathon will require both resistance ability that allows holding those (expected) 80+ miles weeks as well as you as well as your muscle/tendon resistance to keep going at 5:18 pace for 26.2 miles. In your place I definitely have a long term focus of increasing your mileage over the next 2 years to be able continuously handle 80 to 100 mile weeks. I do not think you need much over 80 tbh to do sub 2:18 as long as you can string those continuously with good training. So the focus should be on staying healthy and slowly increasing your mileage without having interruptions to your running. You can obviously cross train with bike if you want to have little bit more aerobic fitness, but the only thing your are really lacking is having your legs, ie muscles and tendons , adjusted to pounding and your economy will increase with the mileage as well. Good luck, lucking forward to seeing you in Trials 2024.