Missing comma in a compound sentence.
A compound sentence is composed of two independent clauses that are connected by a conjunction ("and," "but," "so," etc.):
I was maintaining my speed and feeling really good but when I looked back he was still close behind me.
Problem and cure: A comma should be placed before "but" to indicate the turn from the first independent clause to the second one.
By the way, the word "therefore" is not a conjunction and shouldn’t be used as one. It is an adverb meaning "consequently." This is a fairly common error in student writing.