마라톤 풀코스를 완주한 적이 있는가?

(Great)

마라톤 풀코스를 완주한 적이 없다.

(That's good enough but I think it would be better with this "마라톤 풀코스를 완주한 적없다." I think it more native speaking to me.)

마라톤 선수이 마니라서 체력이 게속 하지 않다.

(I think these are small mistake not a big deal. You know, が＝이, 가 but you should get which one you need to use.)

(아니라서　ないで、）

(계속　つづき）

correction -> 마라톤 선수가 아니라서 체력이 충분하지 않다. I don't get it exactly at the first. I think that sentence makes sense in Japanese but not in Korean. So I would like to recommend 충분하지 않다. 충분 is same with 十分)