What does 一不留神 mean ...还写了不少东西，一不留神写了不少军报头条?
留神 means "pay attention"

一不留神 means " not paying attention for just an instance"

In the sentence,  "一不留神" implies " before I notice it" 


  一不留神写了不少军报头条


"Before I noticed it, I had already written many lead articles in Military News"

You could use "Before I noticed it" or "Before I knew it"