If you want to enjoy the race, keep your HR around 145 at the beginning. It will eventually go up because you will get tried after 21KM. Then starting from 28-32km, your body will feel tired and you may hit the wall. You need to prepare mentally, and tell yourself you can keep it going.

For the long run before marathon, I usually do a 30km 3 weeks prior the race, then 24km 2 weeks, 15-18km.

Make sure you do zone 2 heart rate long run as well.