I’m not familiar with the NYC marathon specifically, but at every other marathon I’ve watched or ran there are markers along the road telling you the elapsed distance. Some marathons will do every 5km, others every mile or every 3 or 5 miles. If you know which distance markers will be posted along the route you can manually create a lap at those places and use that to keep track of your pace.
I’m not familiar with the NYC marathon specifically, but at every other marathon I’ve watched or ran there are markers along the road telling you the elapsed distance. Some marathons will do every 5km, others every mile or every 3 or 5 miles. If you know which distance markers will be posted along the route you can manually create a lap at those places and use that to keep track of your pace.