The secret to finishing or completing a marathon is mileage. Go out there and clock as many miles or kilometres per week. The mileage should be logged for a continuous period of at least 3 months.
Ask any fast marathoner how much they run and you are sure to get something like, upward of 120 km a week. Some log even more, and I am not talking about elites either, just your average Joe who can run marathons in less than three hours.
Unfortunately, we cannot just adapt that theory and start running 100 plus miles a week. You can start small say 20km then progress from 20km to 35, 50, 60, 70 km etc.
So, if you have been asking how do I complete a marathon? Go out and log those miles!