Thought I'd put this here in case anybody else was crazy enough to try the same thing we just did (decide to run a marathon without having ever done more than a 5k, on 3 weeks' notice). Can't promise that my experience is particularly one that you should emulate, but we probably got at least some things right.
So, other than trying the most reasonable training schedule we could come up with, I think I made just about every mistake possible. The training schedule we tried: 3 runs per week with either off days or 1-1.5 mile recovery jogs in between, the first week 13, 7, and 7, the second week 18, 7, 7, and the last week no long run, just 6, 4, and 2 leading up to the race. The long runs were ideally slower than our goal pace and the others a little quicker. It turned out that the 18 mile run cratered because we weren't taking food/water with us, and around mile 10 realized that we weren't going to make 18, so instead we did 13, ate, drank, rested for several hours, and did another 7 in the night in the hopes that it would be approximately the same (20 total) as doing 18 all at once. We also found that most of us missed a couple of the runs for various reasons. In my case, I had a stress-related foot injury about a week before the race, dialed it down as much as possible, and decided to just go for it. Probably not all that wise, as it definitely was a very sharp pain every time I landed. I suspect this is also because my running form isn't very good -- I've been told that especially downhill you should be landing really quietly because it spreads out impact, and we sound more like a herd of elephants. We knew that the marathon we were running had a lot of downhill (one of the reasons it was chosen), so we tried to do a few miles of downhill running in each of the long runs, though we couldn't really get a long, uninterrupted stretch.
On the day of the race, we felt good from having not trained much in the last week, and thought we'd aim for about a 8:45-9:00 pace for the first half, hoping that we could continue at that pace and end up under 4 hours. Three of us with similar conditioning decided to run together, and after pushing through a little traffic at the start, what we thought was an easy pace for us turned out to be a whole lot quicker -- we were running more like 7:45 but we didn't feel like we were working that hard -- I knew that were trying to go out easy, but this really felt easy, so we just decided we should continue until we started feeling that it was a problem, then slow down. I should have known better, since my 5k time is about a 7:20 mile. But, there were some downhill stretches, and I didn't know better, and we did the first half in 1:43, which was almost a full 15 minutes faster than we were aiming for. We made sure to get both water and powerade at every stop (every 2 miles), as well as food/gels every time they were offered. It's shockingly hard to grab a cup and drink while running, especially when you've got the elephant gait going, and I decided to just sacrifice 10 seconds at every stop and actually come to a stop, drink whatever I needed and get all of it in my mouth instead of on my shirt, then go on. Any tips on this?
Shockingly, somewhere after that my calves started cramping up, probably from being bad shock absorbers (oh, and note to self: if they provide a drop bag, remember to put your thick keychain with like 10 keys in it rather than jangling for 26 miles!). As I tried to adjust to that, it meant my hamstrings started feeling the burden instead, and basically my pace was limited by how much I could take before cramping rather than cardio. I did try stopping and stretching for a bit, as well as walking for about a tenth of a mile, and neither helped -- actually, walking made it much tighter and worse when I stopped! I had been getting all of the electrolytes I could, so I think I just went out too hard and my legs couldn't take it anymore. The last 10 miles of the race were pure torture, especially because I was incredibly motivated to run under 4:00 since otherwise I thought I'd end up talking myself into doing this again. Somewhere at this point one of the guys in our group was suffering even worse and decided to drop back, and ended up around 4:22. I made it to a nasty hill around mile 20 before I finally just couldn't keep any sort of reasonable pace, but knew that I could get under 4 hours just by running about 11:30 the rest of the way. And, having said that, I nearly missed. I discovered at the aid station with 4 miles to go that stopping meant I almost couldn't get started again, and just didn't stop in the last few miles. I thought that the last few miles would be fun, because so much of the hard work was done, there were cheering fans, and all of that, but it really wasn't. Even when we turned to the finish with about half a mile to go, I really, really wanted to walk and just couldn't bring myself to do it. My friend went ahead with 4 miles to go and finished 8 minutes ahead of me, and I limped home in 3:51:34, having managed a sprightly 32 minute 5k at the end and basically collapsing two steps after the finish line. Amazingly, I was passing people regularly over those last 4 miles. Even with the postrace massages they offered, it took all of us about 30 minutes to make it the half mile to our cars when we left.
Would I consider doing this again? I swore I wouldn't somewhere in the middle of the race, but ask me in again in a week or two I suppose. I think I was also looking for a test of willpower that would be really hard, and I certainly got more than I bargained for in that regard. Now that we finished, I'm definitely glad that we did it, but if I thought about doing another one, I'd make sure to train enough that the race itself was more enjoyable. The first 10-15 miles were pretty fun, but didn't nearly make up for the last 10 miles or so. But, if anybody wants tips for how not to run a marathon...