Picklelilly, BB, I am doing London in a couple of weeks then NY in Nov as well. My plan will be to take 2-4 weeks recovery after London, then spend a couple of months focusing on 5k & 10k races, before shifting the focus back towards marathon training some time around early Aug. However, while I am focusing on the shorter races, I will probably still include a long run of approx 16M at least every other weekend, so that when I do go back to the marathon specific training I can start my long runs straight from 18M and be comfortable with it. Trying to maintain current marathon fitness throughout the summer is likely to be a big ask, particularly motivationally.