What's the least amount of training you've done? I run Houston every year and just registered so I can keep my streak going. I've been running as time allows, but with my schedule this Fall I can't really put in a full training cycle.

I've done 26 marathons and usually I at least work up to a 16-20 mile long run. Last year I did a 1/2 marathon trail run in mid September, but nothing longer than that before running Houston in January. It wasn't pretty but I still finished in 4:20.

Curious about others' experiences.

houston was my first in '99. my longest run was 7 miles. i ran probably 3-4 miles a few times a week. no training plan. Would not recommend.

Yeah that would be brutal mental beatdown for a first marathon.

I once wrote down a goal to do one before I turned 30. I Promptly forgot about said goal until about a month before my 30th. I started searching and found a marathon in Ogden, UT, on my 30th birthday on a weekend I was going to be in SLC for work.

I've always been a runner but had only ever done a half before that. Over the 4 weeks leading up I did my normal running, 4-6 miles a couple times a week, with a longer run on the weekend, working up to a 16 miler the weekend before the race. Running is a mental game and I figured if I could do 16 I could do 26.

Marathon worked out fine. I remember googling "how to pace a marathon" two days before the race, which was great because I was going to do the opposite of what I read. I don't remember my time but it was respectable and I didn't get injured. Haven't done another one but still run a lot. Maybe one day, but I'll train for real next time.

Both from a physiologic and a personal experience perspective, the answer is zero.

Be aware, however, that your performance will likely be an hour or more worse than what you would have been capable of following a consistent program.

Example: three years ago a college kid I know ran Grandma's Marathon in Duluth in June, based off a few casual weekend runs and finished in the 4:20s. He came back in October for Twin Cities after actually training and finished in the 2:50s and top 100 overall.