Another, think you are being too hard on yourself. FWIW, I also consider myself to be a "plodder", plenty of endurance but not much speed. My experience with the Canova workouts from his "specific" phase is that they are very tough and only do-able within the prescribed effort range in the final few weeks prior to the marathon when you are at your most fit. Think Canova suggests no more than 8 weeks of his specific sessions and to be honest, it felt to me that 4 weeks and a 2 week taper would be ideal. Be interested to hear what you decide.
Cheers marmite.