It depends on your fitness etc. 36 miles is a good distance if there's a lot of technical stuff involved. If it's mainly tracks and fire roads, with not much climbing, then a lot less demanding. Build up, and if it's your first time doing this be prepared to do it at "tourist" pace.
If you're reasonably fit go for it. If it's the autumn one then you've got all summer to prepare, so use it as a training goal. Enjoy. There's a real sense of satisfaction doing stuff like this.