I'm confused by this sentence:
わたしが答える前に急いで川のふちへ歩いていってしまった。
Before I answered he had hurriedly walked to the river side???.
歩いている means walking i.e. progressive form but しまう denotes completion of a verb. This seems like a contradiction. How do these two parts fit together and what nuance is carried by this construct? How would it differ from 歩いてしまった? Thanks.