My big mileage months have never been planned in advance. I know that too many things can go wrong to mess up a goal like that. However, I have had lots of big mileage months, and they have all been "by accident".
My biggest mileage month was 1648 miles (2652 km) in October 2004. The was the first month of a 3-month cycling tour of Australia, and it ended with the Great Southern Randonnee 1200K.
My second biggest mileage month was 1583 miles (2547 km) in July 2002. I rode a 600K early in that month to qualify for the Rocky Mountain 1200, and then I rode the Rocky Mountain 1200 later in the month.
My third biggest mileage month was 1492 miles (2402 km) in August 2003. That was the PBP 2003 plus touring England and Wales.
But I've ridden 18 -- 1000+ mile months between 1997 and 2005 for various reasons, and none were specifically planned that way.
I'd like to do a 1000 mile month again ... maybe next summer ..........