。
I walked only one way.
Literal
I [topic-は] one-way [only-だけ] walk-[went-ていった].
片道 ('one way, single trip') contrasts with 往復 ('round trip'). だけ ('only') marks the limit. 歩いていく ('walk away, walk there') combines 歩く with ていく (the 'go away' auxiliary).