Hiya, I've just recovered from a chest infection which turned into pneumonia - so no running for 16 days (was really quite ill)
I'm also marathon training, my longest run before I got ill was 16.5 miles and was running the same weekly mileage as you.
After the all clear from my gp I did my first run Monday 6 miles which felt good so went out yesterday and did my LSR and managed 15 miles, I actually could have gone further but didn't want to push it. Amazingly I felt great, no lack of fitness or stamina, breathing all good!
So please don't worry,( I was panicking big time about not running) rest up and get better you will be surprised how quickly you can bounce back.
Katy