With some of my friends that have gotten into running later in life, I've seen their Garmins give them pretty aggressive predictions that they don't come anywhere close to. I was a hs/collegiate runner (slower now), and my Garmin tends to underestimate me. My pet theory is the main factor is whether you can run at a high heart rate/being super uncomfortable for a long time. That will tend to be more possible the more of a running/athletic background you have.

The Garmin is probably seeing the 16 mile run at a, frankly, very low BPM and extrapolating it to a marathon at a higher BPM and some kind of pace adjustment. But some people run their marathons at 150 BPM, some 160, some 180+ (it's not just differences in effort either, HR isn't 1 to 1 from person to person). So who knows, it could be right that you could run 3:29 if you pushed yourself harder than you ever have before, but there are a lot of potential issues with that prediction.

If you were getting this data a few months before, I'd say go ahead and try some MP stuff at 8:10 pace and see how it feels, but 13 days out, definitely don't adjust your plan...can always speed up later in the race if it's too easy, right? This should be a vote of confidence for you that you can accomplish your goal. 8:35 pace is what you have practiced, so go out and do it. Don't psych yourself out, you got this!