Essentially,
@CamColt this is what it comes down to. The debate as to whether it is better to teach the body/mind to be able to run for 4-7 hours, or whether training for a longest period of 3 hours is physiologically sufficient. My advice comes from the physiologic standpoint, as little seems to be gained physically from doing exercise longer than 3 hours and the risk of injury rises dramatically. However, I do understand the counter (mental boost and time on your feet).
I personally feel this mental edge could be accomplished by doing more Saturday/Sunday runs if you have the time for it. If you decide to max at 3 hours on Sunday, then you could do a max training run of 1.5 hours on Saturday to reduce the freshness you are coming into Sunday's run with. This means you'll hit the same level of tiredness as the fresh Sunday, just sooner, thereby reducing the amount of time necessary out on the road the 2nd day. However, there is a HUGE caveat with this information. A training plan should be viewed like a symphony. Rather than solely focusing on the brass or woodwinds, we need to remember how all the instruments come together to create the beautiful music we desire. Be careful using what I say and focus on the 1.5 hour Saturday run and 3 hour Sunday run, but forgetting the importance of the rest of the week.
For me, I've maxed at the following...
1st Marathon - 20 miles x1 (3:34) (max per week 26 miles) - Time 4:50
2nd Marathon - 20 miles x2 (3:03, 3:07) (max per week 48 miles, Dopey training) - Time 4:35
3rd Marathon - 20 miles x4 (2:50-3:24) (max per week 31 miles) - Time 4:20
4th Marathon - 17 miles x1 (2:46) (max per week 42 miles, Dopey training) - Time 4:27
5th Marathon - 20 miles x2 (3:14) (max per week 43 miles) - Time 4:58
6th Marathon - 16 miles x3 (2:28) (max per week 60 miles) - Time 3:38
My 6th marathon was BY FAR the best experience I have ever had running a race. Never felt overly tired and finished very easily.