I think the general idea of the postings so far are that if you are only running 1-2x/week, you cannot adequately prepare for a 3:40 marathon in 6 or even 8 weeks.
I would disagree that a 6 week marathon program cannot produce good results under the right circumstances. I ran a 2:21 off of a 5 week program (counting taper), then a 2:16 off of a 6 week program (counting taper). However, I was 22 years old and already had a baseline fitness level prior to those two races.
Maybe you are a young buck. Maybe you are an outstanding triathlete, and besides your 1-2 days of running per week, you are swimming and cycling big volume the other 5-6 days. Maybe your 1-2 runs/week already include a run in the 2+ hour range. If these things are true, then there is hope for an 8 week marathon plan. Let us know if this is the case.
If the above prerequisites are not present, then heed the others' advise and put off your marathon ambitions until you can complete a 16 week plan - at least.
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