Yes, it can be done. Add a little distance onto your long run each week and you can do it. You won't be fast, but it is doable.

Hey Marcia, what is your definition of marathon success?
If it means finishing, you can do it! If you have the option to run more, I definitely suggest that you do, but if not, you can finish a marathon with 3 days of running from your base.
If you stick with 3 days be cautious on race day and listen to your body's needs. I have found that you will probably need a little extra "umph" on race day by means of hydration and fuel since the "memory" is not there in your muscles. I finished a marathon with a light running schedule like yours & experienced muscle fatigue earlier than in other races when I train well. My recovery time was also a little longer after that marathon. Only you know your endurance ability so be smart/safe that is what matters most. Let us know how the 1/2 goes in a few weeks.
Happy running.

As the Nike ad says, Just do it. I used to run only 3, and have run some successful marathons with that. Of course, my runs were all long slow distance in the 10 to 20 mile range. I was able to run in the low 3 hour range with that. I changed to 5 days a week, and lowered my weekly mileage to bring pace down under 3 hours. I always made sure that I took Monday and Friday off from running and lifted those days. The big difference was that I added a weekly tempo run on Tuesday at a pace 20 seconds slower than 10k pace and I built my long runs from about 10 miles to 20 miles over 10 weeks. Every 4th week, I would cut my mileage by 30% by running only 10 miles in my long run. That w as several years ago. These days I run about 4 hours for the marathon and essentially follow the same routine.

Of course you can have marathon success running 3 days a week, but if you want to be stronger/faster, you must do interval work. Long slow distance will only train you to do long slow distance! Do one increasingly long run (or race) per week. Do some speed work one day, mixed with some tempo running -- always with a good warmup and warmdown. I run on a .8 path at a park, because I don't have easy access to a track. The exact interval doesn't matter as long as you are pushing yourself. On the tack you can run 200's or 400's or whatever. Do not overdo the speed sessions!! When you start to lose form, that's enough!! I run the first .2 at 5 minute pace and the .6 at 7 minute pace. The important thing is to run the fast interval as relaxed and smooth as you can run the interval in the shortest time. Don't worry about total miles!Do pace work on the other day -- maybe repeat miles at marathon goal pace. More and more repeats with less and less slower running between. Don't be afraid to take an extra day of rest if you find yourself hurting!! And do race from time to time. You will get more out of a half marathon against competition than you will from a lazy 18 miler!

I ran my first marathon last Spring having trained on the 3-day a week schedule. I have four small children and I knew that was my only prayer. I am not a fast runner and my only goal was to finish. I did so in 4:15. I had no time for any of the cross training the program recommends so I only did the taining runs. I'm sure I will have better times when my schedule allows a more well-rounded training schedule, but having said that, I experienced no injuries and truly enjoyed my entire "virgin" experience. Depending on what you are trying to accomplish, I highly recommend this program for getting your feet wet, having a life and breaking no records other than your own sense of accomplishment. I also used the half marathon 3-day training schedule and completed my half in 1:52. It does work if you just want a respectable finish. Good luck!

I ran several marathon using 3 days per week. All finish times were 3:18 to 3:58, including HSV in 2006 at 3:23. I do these in my triathlon off season.

My plans alway start with 1 long run, 1 day "intervals" and one day of "form" work. Plans usually call for one day of "trash" miles, which I never do. I try to bike, swim or just rest:) The "non" long runs are normally about 45 minutes to 1 hour 15.

Works for me. Has for 8 marathons. I just do not like to push my body any more than I have too.

Weights and yoga will REALLY help you! Do not stop.

Good luck.

One caveat, though. There is some evidence that doing less than 45 miles a week (mpw) while training for a marathon results in greater heart stress durning race than results from training more than 45 mpw. I've done 2 Marine Corps Marathons all on 4 days' a week training and mostly well below 45 mpw. These articles, collected from a set of posts on the Cool Running fora have incentivized me to push up into the 45-50 mpw zone:

http://www.thefinalsprint.com/2007/02/heart-health-and-marathon-running-preparation-is-key/

http://www.news.harvard.edu/gazette/2007/02.01/99-marathon.html

http://www.annalsnyas.org/cgi/content/abstract/301/1/593

http://circ.ahajournals.org/cgi/content/abstract/114/22/2325

http://www.nytimes.com/2006/12/07/fashion/07Fitness.html?ex=1174104000&en=7a8bcad291fb75dc&ei=5070

http://query.nytimes.com/gst/fullpage.html?sec=health&res=9D00E1D91538F937A25756C0A964948260

http://circ.ahajournals.org/cgi/reprint/103/2/327.pdf

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Dave Vause
08 MC Historic Half
07 Frederick Marathon
05, 06 MCM
blog: //community.active.com/blogs/DaveVause
personal web site: www.vause.name