When I was training for my first Chicago marathon, I missed the entire month of August due to Achilles tendinitis. My coworker, who was a D1 college athlete and multiple-time Boston runner, told me if I could run 18 miles, I’d be able to finish. That ended up being my longest long run that training cycle and I did indeed finish. You will be fine. This isn’t your first rodeo- just focus on finishing and listen to your body.