Can I finish a marathon without finishing training?

Question

I'm new to running (casually started last year, running on & off). I've since run a few 5ks & 10ks. My weekly mileage is ~20 miles/week with longest run of 15 miles.

If it makes any difference, I've done all of my training w/out taking anything with me (gu, water, gatorade). I just completed a half-marathon in two hours flat with negative splits and didn't feel much leg pain until the last couple of miles.

My first full marathon is in three weeks. I completely mismarked it on my calendar (weeks earlier than I had initially thought) and also missed a couple weeks of critical training (prior to the half marathon).

I realize I won't be able to run the entire thing but really hope to at least finish by adopting some sort of strategy like alternating running/walking and not be miserable.

Can I expect to finish the marathon?

What should be my walk:run ratio and/or pace to safely finish based on the half?

What sort of training if any can I do the three weeks prior?

EDIT: I've read this post. I'm asking more about how or if I can finish rather than whether or not I should try.

UPDATE: I finished the marathon (on five hours sleep with a long run of 18 miles). I ran most of it. The first 15 were a breeze. Had some ups & downs for the following 6 miles. The last 5 miles were a bit rough but at that point there was no turning back (and I was so happy with the running community's encouragement when we crossed paths!). Felt good to cross the finish line in a reasonable amount of time. It was definitely mind over matter at that point though, I look forward to running another one with proper training and beating my time!

Answer 1

Yes, I think that if you can run a 1/2 marathon, you should be able to at least complete a marathon.

The method that I am going to suggest is called the Galloway Run/Walk program. There is a good PDF article here, that has a chart for pacing and how much to run or walk by pace.

Make sure to read/note this paragraph:

Runners: Remember that long ones should be at least 2 min/mi slower than your projected finish pace in the half or full marathon. An additional slowdown should be made for increased temperature: 30 sec per mile slower for each 5 degrees of temperature increase above 60˚ F. It is always safer to take more frequent breaks.

So using the Cool Running pace calculator (Love this tool), you are slightly over 9 minutes/mile for a 2 hour half marathon. I would then go to 11 minutes per mile for the marathon, which would be 2:30 minutes running, 1:00 minute walking. In theory, that should allow you to complete the marathon.

For the training prior, I would just continue your 20 mile weeks, since there isn't really that much to taper. I would back off on the intensity a bit for any speedwork you are doing, and get a couple of good days rest leading up to the event.