It's a difficult topic to discuss because of the variation from person to person.
I have not used a special block for a marathon but my coach had me do them for 10k usually about 6 weeks out from my goal race. I would do a one week block with a 25% rise in mileage and intensity (not recommended to increase both this severely if you have a small aerobic base). His focus was on working my anaerobic system and Vo2 max. A marathon would require an approach to stressing the aerobic system in a special block. After the one week block we would decrease my usual mileage pre special block by 25%.
Recovery is the most important part of super compensation because your body needs time to recover and adapt to the unusally high stress you placed on it during the special block. If your mileage is under 80mpw. I would not recommend increasing both intensity and mileage at the same time during this block. Either raise the intensity or the mileage.
A special block could last 10 days if you want but overtraining may sneak in if you are not careful at that length. If your block lasts 7 days. You then recover for 7 days for super compensation to kick in.
This is the best I can give you considering I have not used it in marathon training. Another poster may be more informative on it for that.