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...里面"或"与...接触"。 ran into 是一个短语动词，字面意思是"跑进...里面"，引申为"偶然遇见"某人。