There can be diminishing returns on running over 3 hours for some athletes. I would wait until you get closer to that workout and make a call on how you are feeling in terms of resilience and overall fatigue. If you feel good and strong you could go for the distance (as I am sure it mostly be in zone if you are on a level 1 or Level 0 plan). If you feel the body would not benefit from that long of a run then run to time and 3 hours, nailing all of your other sessions.
This is a comment we have in our long run workouts in the plan:
The importance of Long Runs in marathon training is somewhat exaggerated. Research has shown that both average and peak weekly mileage have a bigger impact on marathon performance than does the distance of the longest single run.