I was out for a jog in the park when I ran into an old friend I hadn't seen in years.

ran 是动词 run 的过去式，意思是"跑"。 into 是介词，表示"进入...状态"或"遇到..."。 ran into 是固定短语，意思是"偶然遇见"，强调意外相遇的情况。