First time calling for a ride (marathon advice needed)

This^  You're doing good. Better find out the cause of that blister problem, though. 

If your training has been solid thus far, and I know it has, one messed up week or even two--even NOW--is not what is going to make or break your marathon.

15.5 is plenty to go on for a long run going into the mary, provided your running volume has otherwise been sound and sufficient.

It has. Relax. Taper. You don't need to do another long run. You can always decide to bail on the marathon later.

I'm going to say go ahead and run the 20 this weekend.    Unless you have reason to believe it will be a fail.   Confidence is the key here.     The 3 week taper thing I don't buy into exactly.   I think 2 weeks is just fine.    In fact I'm also running 20 on Sunday and the mary on Oct 9.     Here is the thing.  even if you have to run this on a treadmill due to weather or just to plain make sure the run is a success  then do it.    That success is going to be crucial going into the marathon.     Do you need it physically?    Probably not.   but maybe.    If your legs feel really beat up  then don't do it.    it has to be a successful run or there is no point to it.   If you need to take a few days off before that 20 miler  then do it.    But then everything rides on that run.  

Unless your legs feel bad,  then that 15 miler will help you that much more for your 20 miler.     I'm thinking you can do it.

I agree that a 2 week taper is enough.  But it all depends on you.  I'd need the long run (18 - 20)  just for my confidence.

good luck,

Duane

About those Blisters.

A few weeks ago I raced two OLY distance events 6 days apart.  During the first race I decided to bike and run without socks.  I didn't feel any discomfort during the race, but after my cool down run, my feet were wrecked.  Quarter size blisters on the sides and heals, dime sized blisters on the outsides of my toes.  After cleaning and draining I put liquid skin on to protect the loose peice of skin covering the blisters.  I reapplied the liquid skin each day and was able to continue training.

Come race day, all but one of the blisters had healed completely.  The loose flap of skin reattached itself and there was no discomfort at all.  The Largest blister on my left heal was very small.  I raced the second OLY with shoes and socks with nothing else protecting my feet.  The race went from something I thought I might not be able to start, to a finish with 17min PR.

Protect your feet and they will carry you to the finishline.

I had an almost identical situation last year for my first marathon.  I was advised not to worry about the 20 miler, so 18 was my longest.  I just continued with my plan, which was also a 3 week taper.

As I've been told, your performance will be largely based on the total training you did, not the five miles you didn't do or the difference between a long run of 18 or 20.  That's all mental and confidence related.

Good luck, I'm sure you'll do well!

I'd do the 20 miler...even though I'm a believer in a three week taper for marathons, I'm a bigger believer (from painful personal experience) in getting my long run built up to 20 miles (or 2:45+, if you like to think in those terms), if possible.

(E.T.A.  ....but make sure you deal with the blister issue!)

^^^This^^^

A 3 week taper is only necessary if you're carrying a really big training load.  Just a quick look at your logs gives me the impression that your volume hasn't been large enough to justify 3 weeks.  You should be able to recover and peak with 2 weeks.