On April 20th, Mary Akor ran 2 hr 39 min 34 seconds to place 19th at the Olympic Marathon Trials. On May 4th, just 2 weeks later Mary won the BMO Vancouver Marathon in 2 hrs 37 min 52 seconds.
On April 12th, on a spur of the moment decision, I jumped into the Moab Marathon and ran 3hr 7 min. Three weeks later at Vancouver I ran 2 hr 56 min 36 sec.
First of all, let me say emphatically that I do not endorse running a full marathon just 2-3 weeks before your targeted race. I do believe, however, that there is some value to putting in a hard effort at the T-minus 3 week mark. If you look at this study on downhill running, there appears to be a 3-6 week post workout window that optimizes a protective effect against muscle soreness and tissue damage.
From personal experience, I think an all-out half marathon 3 weeks prior to your marathon is the perfect scenario to minimize tissue damage and muscle soreness on race day. You'll tear down muscle tissue causing some soreness, recover, and go into your race even stronger than you were just 3 weeks earlier. I would be mildly surprised if your legs don't do a much better job handling the pounding compared to any previous marathon attempts.
A time trial also affords the opportunity to give your shoes and other racing gear a dry run. Use this race to go over your pre-race rituals and racing strategy including what to eat the night before, when to get up, how much to drink, eating and drinking during your race, and to get a feel for what kind of pace you can handle.
If there are no half marathons on the schedule 3 or 4 weeks prior to your marathon, plan for and do a solo time trial on your own. It won't be fun, but you certainly won't regret having done it come race day.
(c)Dave Elger 2008 All rights reserved