I've ranned 72 miles per month for four years is this enough to run a marathon?

You are averaging 18 miles per week. That is enough to begin a marathon training plan, which will likely last 4 months or so.

Most people training for a marathon try to hit 20-22 miles for their weekly long run once before they start to taper and recover for he race - that's more in one day than you are doing most weeks.

I recommend you Google some marathon training plans (the ones at Runners World are pretty good, so are the ones by Hal Higdon) and read through the kind of running you need to prepare for a marathon. A marathon is a big goal, but it is something that you can accomplish. It is always good to gather information before embarking on such a large task.

Every day = <2 1/2/ day. You need 26.2 miles in 1 day, ( winners run in < 3 hours).