On the treadmill with fresh legs, I didn't need to walk. I just ran a pretty slow pace, varying it to keep my HR at about 75%. (Working a marathon hard, I can hold an average of 85%, 82% for a relatively tough 50k, and a decent but not too hard marathon effort for me is about 80% HR average)
I took a couple short walk breaks to make sure I got hydrated and to eat a gel at one point, but that was it. And the couple minute break to roll out the knot in my calf. In my trail marathoning, with fresh legs, I can usually tick up 15% grades at a 12-13 min/mile pace provided the footing isn't too extreme and the altitude isn't too bad. I'm very tall though, and those levers are tough to move, and once it gets much steeper than that, I cross over to walking. And, of course, a 10%-15% grade later in the course is going to send me to walk mode unless it's pretty smooth (like the final big hill climb in the Blue Ridge Parkway Marathon, which is steep, but paved.)
Thanks for the advice and encouragement on this thread. I'm feeling a lot less nervous. Sleeping in the stupid altitude tent that takes up half the basement on a crummy air mattress with the 02 level set to simulate 11,000 is annoying, and I don't have great faith it'll do much for my performance, but I think it'll help stave off nausea and headache at least given I live at sea level. I'll also be doing an actual taper this time around, something I usually don't do very well.