"As I think about my own approaching marathon it makes me wonder, should I trust clocks on the course or my own watch?"
Runner's World wrote an interesting article on this. In my experience it is rare to run only 26.2 miles in a big marathon because it is measured on perfect tangents and those are difficult to run with the quantity of people and without course knowledge. Your watch will give you good splits for how far you've actually run, which more accurately reflects your pace than the clocks on the course, but don't be surprised when you hit 26.2 miles on the watch and still have a quarter mile to go.
"As I think about my own approaching marathon it makes me wonder, should I trust clocks on the course or my own watch?"
Runner's World wrote an interesting article on this. In my experience it is rare to run only 26.2 miles in a big marathon because it is measured on perfect tangents and those are difficult to run with the quantity of people and without course knowledge. Your watch will give you good splits for how far you've actually run, which more accurately reflects your pace than the clocks on the course, but don't be surprised when you hit 26.2 miles on the watch and still have a quarter mile to go.