If you have been running 35 to 40 miles per week for 1 to 2 months, you have a good endurance base for running a marathon after 3 additional months (12 weeks) of training (examples below). To run a marathon, you must complete some long training runs in the weeks leading up to the marathon. The week that you run the marathon, however, should include only a few short runs. Your goal for your first marathon should be to complete it.

A 12 Week Marathon Training Program Starting at 40 Miles/Week
Week	Mon	Tues	Wed	Thur	Fri	Sat	Sun	Total
One	-	5	8	5	5	4	12	42
Two	-	8	5	10	5	5	12	45
Three	-	6	8	6	6	4	15	45
Four	-	8	8	10	5	5	14	50
Five	-	6	8	8	6	4	18	50
Six	-	6	10	12	5	8	14	55
Seven	-	5	10	10	6	4	20	55
Eight	-	8	10	10	10	8	14	60
Nine	-	8	10	10	6	6	20	60
Eleven	-	5	10	10	5	4	18	55
Twelve	-	8	6	4	-	2	Marathon

Other Points to consider
- Make sure you eat enough carbohydrates and are well hydrated before the marathon
- Start out slowly and pace yourself
- Walk for a while if you get cramps or feel fatigued
- Consider the environmental temperature
- After you finish, stretch and walk around. Take a hot bath.
- Slowly return to running

Article Ref:
Navy Seal Physical Fitness Guide