I've never worn a HR monitor for a marathon, but here's a treadmill workout I did two years back with one one that was 2x5 mile at somewhere between 5:45-6 min/mile pace (goal marathon pace that year was 5:42). Ignore the pace/mile splits since the accelerometer is very inaccurate on the forerunner watches


https://connect.garmin.com/modern/activity/426033457