I've done this flight four times on UA. There's no getting around the fact it's a long time in a seat.
Go for EC and if you can get a bulkhead or exit row. If you want to spend the money on premium economy to for Virgin.
Bring snacks on board so you don't have to rely strictly on meal service.
A good book helps.
You will enjoy Australia. Awesome place to visit. Try to get out of the Sydney and see more of the country.