1st Time Marathon Advice Needed (please)

so i've found my race. i have a plan. now what? i'm approximately 30 weeks out from my race. i found a 20 week intermediate (the beginners seemed too easy...) training plan at cool running. what do i do for the next 10 weeks? i've been running approx. 15-20 miles a week for the past month, long runs 7-13 mile range. am i correct in presuming i'm fit enough for the intermediate plan or should i back off?