Nine weeks until taper for London

The London marathon is in 12 weeks, which gives me nine weeks to build up training before the taper. Training is going OK but not great. I'm up to 18 miles on long runs but not as fast as I'd hoped. I'm thinking a sub 3:00 is unrealistic, but a sub 3:15 is plausible. A sub 3:15 would allow me to qualify for London 2013, assuming "good for age" requirements are the same as they were for this year's race.