How far do you actually run in a marathon

Have NYC in a few weeks and my goal pace is 9 min 9 seconds bringing me to 3hr 59 min 54 seconds. I did a 10 mile race on Sunday as a practice and I noted that I actually ran 10.1 miles and I was sure to start / stop my Garmin Fenix 7 at the exact point. I list my watch as it is relatively new so I suppose accurate? For some friends they also ran over the distance with their apple watches and theirs were like 10.1-10.25. I think this is from weaving in and out of people. Extrapolating that for a full marathon and I think I could run closer to 26.5 miles? So I need to run faster to keep to under 4 hours? What should my goal pace be then for sub 4 hour?