Your friend is mistaken. A three-week layoff will make only a small impact on your fitness - probably almost none if you're running three times a week. In fact, if you were feeling tired at the end of your most recent century, I'd say a short break will do you good.

And with regard to the running, are you a regular runner? If not, take it easy - running uses the muscles very differently from cycling, you'll be stiff as a board.

PS. Only someone from Florida could call a century with 4200 feet of gain a "horrible hundred". That's a moderately flat ride!