ideal taper length?

running my first marathon (chi) and doing hal higdons intermediate 1. i am a week behind where i should be (did 18 last week, 14 this weekend). next weekends are 20,12,20,12,8 i’m wondering if i still want to hit 20 2x before the marathon, if 2 weeks before the marathon is too soon to be running that second 20. thinking i would still aim for it - but if that’s foolish i could just skip that second one. thanks for any advice!!