[1020] How to render new added page without removing previous rendered ones

I have over 3000 post, i'm rendering all of them - it took like a 20 mins. After this i'm adding new post to my db and i need to render this one page to join it to just rendered files/posts. How to do this without removing already created data? I wish to use some process env like: GENERATE_POST so my generate block will looks like this:

generate: {
  routes: function () {
    if (process.env.GENERATE_POST) {
      // generating only one post
      return ['/feed/' + process.env.GENERATE_POST]
    } else {
      // generating all data in promise loop
    }
  }
},

Thx for help.