Hi Jan I wouldn't worry about going long haul for your first JY holiday - if Australia is your dream you should go for it! I'm strongly thinking about it for 2015, if I can afford it by that stage!
I had similar feelings to you when I first discovered JY three years ago, so I decided to book a week in Andalucia to see how I enjoyed the JY experience. Whilst I had a great week, afterwards I thought to myself that I should have just jumped right in and booked one of the long haul tours I really wanted to do! Since then I've done the US (twice), India, Vietnam and Cambodia, South Africa/Zambia, South America (with Travelsphere) and I have two more booked - Costa Rica/Nicaragua and China!
There is really nothing to worry about travelling long haul. When you arrive at the airport, find where you check in and you will most probably meet some of your fellow travellers there (as well as your tour manager or JY airport rep). Even if you arrive very early and there's no one else there, after you have gone through security you will no doubt meet someone from the tour very quickly. As Dave said, you'll probably be seated with some fellow travellers on the plane - and even if you're not, there will be plenty of in-flight entertainment to keep you busy and hopefully you'll get a few hours sleep as well! So I do hope you follow your dream and have a wonderful holiday.
All the best,
Bob