It's a pretty solid progression. Did you do something similar when you ran your PR? I recommend in either week 17 or week 18, you should do a specific type of long run. It's tough, but I've run some solid PRs every time that I've done this, and so I do it before at least one of my marathons every year now. Here's an excerpt from my book on marathoning where I describe the workout (easier to cut & paste than paraphrase):

The last "Long Run" before your marathon I have one specific workout that I try to run between 3 and 5 weeks before my marathon. This workout will change from year to year, and there are different ways of accomplishing it. The general idea is that I want to make myself tired and exhausted, and then go for the long run. My favorite workout of this type was in 2006, when I raced in a 5k cross country meet at a local park. Once I finished the race, I spent a few minutes to grab some water and food, chat with the winners, and then I ran home. The route that I chose was 16 miles with a not insignificant net elevation gain. I ran between 20 and 21 miles that day between my warm-up, my race, and my long run. If there is not a convenient cross country race that you can use to begin your workout with, I find that a fairly simple way of simulating the end of a marathon is to run a 10 to 12 mile tempo run at marathon pace on one day, and then to run your long run the next morning. It is probably a more controlled and easily managed method for getting this type of workout in. On two occasions, I have run longer than a marathon distance for this workout. While the strategy may work for some, especially if you are an experienced marathoner, I would not necessarily recommend it. The first time was in 2004 when I got lost on what should have been a 17 mile run, whereupon it turned into a 28 mile run. I believe that that run directly contributed to a pulled hamstring a short time later that was still bothering me when I got to the marathon. The second time was when I ran the Pisgah 50k Mountain Race about 7 weeks before running a marathon. This did help with the marathon, but it hurt my 5k race performances that fell between the two longer races.

This year, I'm running 2 marathons in the Spring, March 30 and May 4. The March 30th is going to be a marathon-length tempo run that I am going to plan on running in about 3 hours, since I'll be spending the two days before the race sitting on my ass in a classroom and I had decided to run the new marathon in May. What I've been doing is getting my mileage up and running my long runs on the trails. Since I live in Maine, that means that I've been doing them in the snow. Usually, it's not too bad. I run on snowmobile trails, so with screw shoes its actually easier than running the trails dry. There are also days like this past weekend after getting 10+ inches of snow. The snowmobiles packed it down somewhat, but there was still a few inches of loose snow to slog through and as it got later in the day and the sun came out it all got soft. Since we start the run at a mountain, it means that the end of the run is uphill. Good thing I train with guys that do 50 and 100 mile races - watching what they do makes it easier for me to suck it up and think about what great strength training 3 hours on a snow filled trail can be. The end goal of course is going to be a PR on May 4th in Rhode Island. Right now, I'll be content with anything below 2:45, since a couple of my teammates ran a 2:45:37 last year a few weeks after I ran a 2:48:03. My goal is probably going to shift down, though, since they are both running in the Olympic Marathon Trials and I really hope that they better their times there. Whatever they run, though, I need to beat. That's how I'm setting my goals for this Spring. I haven't actually mapped out my long runs. I've already got 2 at 18 miles, and will probably try to get at least that every week or 2.

This sounds like too much the last three weeks before your goal race: > week 19: 36k > week 20: 29k (23 at marathon pace, 10k race) > week 21: 20k (12k race) Week 19 would be good, by itself. Week 20 might be OK if it wasn't between your longest run, on week 19 and another, longer race on week 21. Running 14 miles at marathon pace is going to be tough. Racing a 10K after is going to be tough. Week 21 sounds ill-advised to me. Sure, you feel fine after a 10K or a 12K. If you run one after another, week after week, you'd probably notice your times getting slower, not faster, though. Week 21 still might be OK, depending on your recovery time, if not for the preceding two weeks. Put weeks 19, 20, and 21 together, and you are destined to be worn out at the starting line of your marathon. How did you come up with a plan like this?