How important is a "long run" in half marathon preparation?

[Sasha Pachev]

A hard half + some warm-up and cooldown will do more for your marathon conditioning than a 20 mile jog. There is even some science behind it. When you jog, you burn fats. When you run hard you burn carbs. When you run hard for a long time, you deplete the muscle glycogen, which encourages the body to store more of it in the future.  When you bonk in the marathon, it is never because you are out of fat (even if you have less than 5% of it on your body), it is because you are out of glycogen. The more muscle glycogen you've got, the  longer you can go at a good pace.

Also, who is to say that a long run or a hard carbo-depleting tempo ideally needs to happen exactly once a week for everybody? Why not once every two or even three weeks for an older runner with a busy schedule? And if she tries it more often, she will probably overtrain.

[Josse]

There are definate benifits to running 6 days a week and I think it can carry you through a half marathon just fine.  But I also see the value in the long run.  It gives you peace of mind that you have gone that far in past 3 months.  It also builds endurance, you can pracitce your desired pace on tired legs which I think is benifical.   And I do think if you are looking to better your times you need some long runs.  Anyone can finish a half, it just depends on if you are running it to walk or running it to get a PR. 
I don't think you need a whole lot of long runs but at least 2-3 through you training, unless you are wanting  a time in the 1:20-1:30's.

[Lybi]

Michelle--When I said "this is all Sasha's fault" I was of course joking.  The truth is that I was just still very middle of the road on my decision and he left a long comment on my Tues. (June 10th) blog that helped tip me over to the other side again.  Here is my reasoning... 

One of the biggest obstacles I've had since starting to run is getting my weekly mileage over 30-35.  For some reason, every time I have tried to go higher than that (and I know it is not that high) I have gotten injured, or suffered a "lapse" in motivation which I think is related to feeling tired all the time.  I am sure that long runs are beneficial.  The ideal training regimen in general would be high weekly miles AND some long runs AND speedwork.  But which is the most critical to increase first?  I am voting on weekly mileage.  I REALLY REALLY want to be able to get up to at least 40 miles a week.   But I need to be sneaky to get there.  I really have to try to minimize the stress on my body as I can to avoid pushing myself past the injury point (and also the "Why do I do this--I hate running" point).  Then when I am there (comfortable with, say, 42-45 miles a week) I can hopefully add a regular long run, and then eventually some speedwork.   Anyway, that is my plan--for now.  Let's see if it works better than what I've been trying.

[Josse]

I think you have a perfect plan to get you to your end goal.  I have allways believed in building slowly to aviod injury.  And I still think if you run 6 a day you will be able to finish a half just fine and even pr.  You better or you will make Sasha look bad