It is busy for the first 10km really as you make your way up in to the hills. Not so bad that you can't make your way through the 'crowds' if you want to. There was/is one bottleneck where you go through a narrow gap in the woods and then through a kissing gate. There are a couple of other pinch points where you can only really run two abreast, but otherwise you should be able to find and keep your own pace.
I ran it last year with my wife. It's easily the best event I have ever done for a whole host of reasons. I am sure you will love it.