Robert, I did a 3:04 at FLM this year off 3x 20m runs (last one was a 20m race with the 2nd 10m @ MP) but to be honest the only reason didn't do more was that it was my first marathon and I was building from scratch. I was in pretty good nick generally although I wouldn't say my endurance form was optimal. HR drift started occuring about 21 miles and pace dwindled siginificantly after 22-23.
As it happens, I have scheduled myself to do 6 this time round as TT suggests, and I'll decide how long to make the longest (somewhere between 22-24 I imagine) depending on how my legs recover from the first couple.
And obviously this is a sensible time to be asking the question - the more 20+ LSR's you're planning to do, the sooner you'll need to ramp up the mileage. With taper and cut-back weeks I'm due to be up to 20m before end of Jan (for Paris, 1st week in April).