I have received corrections concerning this a few times now, so I figured I would write it down once and for all:
Generally the topic marking particle 은/는 is used when expressing a negative.
저는 절대로 마라톤을 안 할 거예요 ~ I will absolutely not run a marathon
In this situation I initially thought of using 제가 instead of 저는 as if to say that others can run as much as they like, but I will not join them. However I was corrected to use -는 since it is a negative sentence and -는 then sounds more natural.
Noun + 아니에요 takes 이/가
커피가 아니에요 ~ (that) is not coffee