Shortest long run for marathon preparedness?

Andrew Starykowicz only did 14 miles as his longest run before going out and winning the Rev3 Cedar Point Full last year. Then again, he's a pro.

As for me, I've never done one. But I'd put it out there that 20 miles would make me comfortable to just go out and finish.

In all seriousness, that depends on a lot of factors.  How good is your base, what experience do you have with distance running, are you willing to walk (maybe a lot) and finish just under the course cut off, etc.

I would say that most plans would have you run at least 18 and some up to 24.  I'm running the marine corps marathon in a month and I haven't ran farther than 13.1 miles since last November.  I have no illusions that I'll PR or even do well by my own standards, but I'll finish the race without a doubt and it won't "kill" me in any way shape or form.

like someone else sad, there are a lot of factors.

i'm on the fence of signing up for one in 3 weeks, 1 week after my last tri of the year.  my longest run will be 18 if i do it.  the course will be mostly flat though, and with my last 15 mile run having 1800ft of climb, I'm not too worried about it if i decide to do it over the HM.

Do you mean the shortest long run in the progressive sequence of training (IE 8, 10, 12, 14, 16, 18?)

Or do you mean to ask how long your longest long run has to be to run a marathon?

21 or 22.

but it's much more about your total weekly mileage than the length of your long run. 

18, due to some injuries.  Finished ok, but definitely preferred it when my long runs were over 20.

I love the way you think

Did 20 miles - best run of my life - nailed it perfectly, negative split and all. I ran the last 6 at a smok'in fast pace, at least for me that is. Ran the marathon 3 weeks later and the last 6 miles were some of the most difficult miles mentally and physically I have ever done. By far the hardest race of my life, I never want to repeat that experience again.

When I do it again (mind you a few months ago I would have never said "when I do it again") I will most likely do a 22 mile run and not do such long taper. It was my first and a good learning experience - but a pretty big learning curve.

PS The day I did the 20 miles I could have done 26 easily........Well I think I could have.

I never ran more than 13 miles before the first marathon I did. I was 30 at the time.

I have run 9 marathons since then and 3 IM distance races. My long runs vary, I usually try to get at least 1 18 miler in, but as someone else said, it is about weekly volume and the type of prep you do.

I find I can get by on 35 miles a week as long as I do some tempo based runs.

Then again, I have yet to go sub 4:30.

16 miles - asking for trouble, unless done with a huge mileage base

18-20 miles - probably ok, and quite standard; overall volume is also important

22+ miles - if it doesn't kill you, then you're probably good to go

16, but it was a month after an HIM build.  I don't think I ever got over 120 miles in a month though.

I will say this, miles 21-24 were UGLY.....then I started hallucinating and it was fine.