First Marathon - Question regarding long run pace

I am currently following the Asics Target 26.2 sub 4 hr marathon plan in preparation for London 2017.  Everything going well so far.  However, I have entered the 20:20 Fusion 20 mile race on Saturday March 11th.  The training plan states to run '20 miles at 10 min/mile pace' on that session.  Being a race, I was wondering if it would do any harm aiming for marathon pace for this (9 min/mile for me) - I would like to do this if possible.  My mileage / endurance was pretty good before starting the training plan and have completed numerous half marathons and runs of between 14-18 miles  There is another 20 mile long run in the plan a couple of weeks later which aims fort a negative split.  Any advice gladly received.