Well that depends on a few things:

• What is your marathon goal?  To finish or to do it as fast as you can?  Or something in between.
• Are you already a runner and if so, what is your "base" mileage (i.e. typical weekly mileage).
• How far off is the marathon?  Typically it is good to give yourself at a bare bones minimum 6 months to train for your first marathon, though 9 to 12 months would be preferable.
• Do you have injuries or are you prone to certain injuries?

I've known people that can get away with as little as 30 miles per week for their marathon and I know competitive types that feel they need to peak at 80 to 100 miles per week to get a fast marathon time (that used to be me when I was young...I'm way too old for that now).

We are talking here about "peak" mileage.  If you are a marathon novice that currently does maybe 15 miles per week, I think you should target gradually increasing that mileage to a peak of at least 30 miles per week.  Your peak mileage generally should occur 4 weeks to 6 weeks prior to the marathon.

But you know what...everyone is different.  There are no hard and fast rules.  Everyone is looking for one, but when it comes down to it, you've gotta ramp up your mileage and training over time, try not to get injured, work within your personal time constraints (e.g. work, relationships, family, kids, hobbies, reading Conejo Valley Guide, you know important stuff) and do your best.

That said, I've always told people to try to peak at a minimum of 30 to 35 miles per week and to run at least 4 days per week to get that mileage in.  And most importantly, your "long run" (that I will no doubt comment on much more at a later date) is the most important component of your mileage increase.  I tell people that to feel "comfortable" at the marathon your longest run(s) should be at least 20 miles.  But we'll get into that more in a separate post.

Your Goals Can Be Re-Set.  I remember it well.  Barack (not his real name...but I do know of one Barack) started his marathon training at probably 240 lbs at a height of 6'.  Most of that weight was in Barack's belly.  Barack was a busy executive who didn't have a lot of time to train, but he took it seriously.  However, 2 months before the 1996 Los Angeles Marathon, he confided to me that his longest run was only 10 miles.

Barack was determined to run his first marathon.  I told him that if he could ramp up his long run to at least 13 miles (half the marathon distance), I think he could slog through the whole marathon.  I could have taken a harder stance and told him to do at least 16 miles, but I knew that wasn't possible for him at that point.  He would probably injure himself.  He needed the mental encouragement more than anything.  He could finish the marathon indeed...but he may have to walk/jog a good portion of it.

Sure enough, 2 months later Barack was ecstatic.  He finished his first marathon.  He was thrilled to earn that medal on his neck.  It took him 6 hours or so to finish the race, but he did it.  I don't think his peak mileage was more than 25 miles.

The key thing is...get on out there, take that first step, and start doing it!  Don't belabor the details.  Just get on out there and start running!