G'lord! The Mount Marathon Race sounds intense! I also looked it up and it's a climb AND a descent on Mt Marathon with cliffs. Oh, but there are waterfalls and a spectacular view. I'm not physically fit but WOW, if I was, I'd be on that piece. I say this because people of all ages have run, even women in the 50s and 60s.
I say, go for it and good luck!