Lose muscle m on my legs and arms?

I am a cross country runner, but every track season i work my legs hard using weights and sprints. i used to weigh 110, i bulked up to 118 this year, and i don't want to lug around all of this muscle during XC. the only meat i eat is fish, and i run almost everyday, and i am 5;7" tall. As soon as track is over i want to start getting my legs leaner again. How can i lose this the fastest?