"You are running too much."
Translation:Sen çok fazla koşuyorsun.
In other words, just to make this a bit more clear, "Sen fazla koşuyorsun" (without the "çok") means "You are running too much." The use of the word "çok" simply adds emphasis and like Ektoraskan said, you can omit it.
The word "fazla" has many different nuances in meaning and can be used as an adjective or an adverb (as it is used here). To see a list of the different meanings for "fazla," click on the link below:
You'll see that one of the definitions is "excessive."
Anyway, good question. I am glad you asked it and hope that, in addition to the other posts, it was helpful in some way.