Mile-stone or mill-stone?

I realise that when I give updates on the word count of my novel in progress, the posts aren't as thrilling as readers might like. However, there's an enormous sense of achievement in putting together words that make sense after a writing session and therefore, I'm very happy to say, the novel is now up to 21,000 words.

I feel that it's a milestone of sorts as it's approx. a quarter of the way through. All I need to do now is put in the time to repeat those 21,000 words another three times and I'm there, all done.

What I'll do then? I haven't a clue.