The Boston Marathon in scheduled for April 20. Because of an injury I worked through, I did my first 20 mile training run just yesterday (March 22). I would like to fit in one more 20 mile long training run before the race, but is that wise? I am tentatively planning on running 16 miles next weekend (March 28), and then a final 20 mile run on April 5, which would be 2 weeks and 1 day before the race. Is that enough time to recover? I have been logging about 35 - 45 miles per week.
Some swear by a two week taper, others need three weeks. How well have you been recovering from your long runs? Have you done them on consecutive weekends or do you take a shorter week between? The answer should help you decide on how long a taper you should take. You also could do 20 tomorrow instead of 16. The additional 20 would be nice but not absolutely necessary.
Another alternative...instead of trying to "cram" in a second 20-miler, consider looking at your weekly mileage instead of your long-run mileage. Is there another way you can do another high-mileage week WITHOUT a 20-miler in there? For example, make one of your short runs into a medium-distance, slower paced run. You won't lose any speed at this point by backing off of your tempo or pace runs for the last 3-4 weeks. In fact, that may be a good taper strategy anyway. So, I'd suggest you consider stretching out the distance of one or two of your midweek runs, instead of trying to cram another 20-miler into it.
That being said, I'd also echo what was already said: how have you recovered so far from your long runs? I've experimented with my recovery and found that when I have a good base of regular mileage, I don't need as much recovery time as all the books say I do. Case in point, last year I ran 3 marathons in 7 weeks...the third of which was just 7 days after the 2nd marathon...and I ended up finishing 2nd in my age-group, 30th overall...my first "podium-finish". So, now I've reduced my pre-race taper to 2 weeks, and have cut 35-minutes off my PR. Granted, I've experimented a lot, and have the experience of 8 marathons under my belt now (with very detailed training journals for all of them...which I review like a true running geek). Anyway...what's your body telling you? One thing I've learned is that you've gotta learn to listen to your body before listening to the piles of well-intended, but not-customized advice out there from folks like me.