Most marathon programs have you run a 18 to 20 miler three weeks before the event. This gives you enough time to recover as runs that long cause micro tears in your muscles that need to be healed. For my first marathon I ran one 20 miler three weeks before. I am running one next weekend and I have ran two 20's and a 22.

What I would recommend is to run these long runs in the same amount of TIME you plan on running the full thing in. For example if you want to run a 3:30 marathon then do your 18 to 20 miler in that amount of time. You want to be ready to be able to run that long of a time. The feet time is the most important part for me. I worry about putting in the speed during the marathon. Good Luck and I hope this helps.