Crazy Challenge: Sub-3 in Sub-3?

I haven't run a marathon for a while. While I wanted to qualify for Boston Marathon 2011, an injury (I fell on a jog in Davos, Switzerland, and badly twisted my ankle in mid-August, apparently it took more than a 1.5 months to recover) kept me from any serious training. I tried to jog every now and then but I didn't run much more than twice a week for an hour or so. It's not that I didn't exercise at all: I'm cycling daily and I'm using some of my muscles during yoga (which I just started).

Time is ticking. Registration for the marathon started today. The question is: can I get back in shape to qualify within three weeks? I am definitely not eating as healthy as I did in Japan; a tight budget adds an extra incentive to eat whenever there's free food anywhere (yes, we're talking pizza and cookies).

Here's the challenge:
The time left is a bit less than three weeks until November 7. The (very) optimistic goal is to go under three hours. The hope is to go under 3:10 (which is the qualifying time). And another hope (which is not under my control) is that there are still spots available by that time.

Sub-3 hours in less than three weeks?
Any advice? How should I train?
Who wants to take a bet?
(I actually don't believe that I can do it yet... I hope to know more next Sunday)

Some statistics:
Current weight (using the scale in our bedroom, don't know whether it's accurate): 155 lbs (a bit more than 70kg - damned brownies...)
Previous runs:
last Thursday: three laps around fresh pond (1h)
Saturday: four laps around fresh pond (1h20)
Sunday: jogged to yoga and back (25 minutes per way), ran stairs in the Harvard stadium (half-way)

Going for a jog now...

PS: the reason I need to re-qualify is that only times after September 2009 count. My last marathon (the disaster in Beppu was in February 2009)

PS2: needless to say that I will also have to work hard during these three weeks, no time to focus on running only.