It's not far at all. London to Istanbul (Turkey) takes 4 hours - if you've come from Australia, it'll feel like nothing ;)

Check on [kayak][1] for lots of cheap EU flights.  And also [RyanAir][2] and [EasyJet][3] for low cost flights - although always check what airport they use, as they quite often use unusual or out of town airports - their flight to Vienna lands in Bratislava, Slovakia - in a different country! (Although it's an easy bus ride to Vienna).

Speaking of Bratislava, it's a cheap hub which quite a few low cost carriers fly to.  You're in Paris - I have personally flown from Orly Airport (south Paris) to Bratislava. It's an easy flight, great view over the alps depending on route, and it's convenient for Eastern Europe.  It's 1 hour to Vienna by bus or 2 by boat.  By train you can get to Budapest, Hungary in a couple of hours, and there are trains from there down to Romania, Bulgaria and Turkey - the route we took.

In Vienna I met people who had come from the Czech Republic by train as well, and we also met another friend who had trained in from Germany in a matter of hours.

In general the European trains are very good, reliable and extensive - you can certainly look at trains from Vienna south towards Slovenia and Croatia too, and I know from experience that the buses in Croatia were very comfortable and relatively inexpensive, and on-time.

So long answer short, it's certainly not too far, but I'd pick a few places that really appeal and spend time in them, rather than rushing around too much.

  [1]: http://kayak.co.uk
  [2]: http://ryanair.com
  [3]: http://www.easyjet.com