When planning a tour, I start with microsoft routeplanner - not the best software for a lot of things, but the version I've got (not the current one, unfortunately), allows me to put in a mph. It then shows daily stops. I then check out cycle routes on the internet (very good in Germany). I then send off to the regional tourist boards, to get as much background info. I check the towns I'm looking at staying at on the internet to see what the hotel situation is like (credit card touring for me, I'm affraid). I then post lots of requests for info on various websites. I've only toured in Germany, but luckily, I speak some German and find I can get great help from a German touring website. I check past weather for the area, so I know I'm not setting myself up for a wet of windy tour, then I dream and dream and dream until it all comes together.
Because for all the worries about how it can all go wrong, when it happens, it's always the best thing in the world.
I have to say, though, the more you plan, the luckier you get.

Off the top of my head (and not know where in Austia you intend going), I'd cycle to Dover, get off at Calais, go down towards the Ardennes, over towards Colgne, down towards Frankfurt, then towards Munich. From there you have to go south and over the alps (though there's a pass I forget the name of that's not too high, then down towards the Inn river. you can then follow it east or west to wherever in Austria you want to go. Alternatively, if you're heading to Western Austria, go down towards Freidrchshaven on Bodensee and cut into Austria there, or if you're heading for Eastern Austria, head west past Munich towards berchtresgaden and then dip down into Austria through Salzburg.
If it were me, I'd go down the Mosell from Cologn, which is beautiful, head down towards the Black Forest, which is also beautiful, then pick up the Bodensee to Konigsee radweg, which is pretty flat, but, by all accounts (I've not done it, but dream of it) beautiful. That will take you to Salzburg without any climbing (climbing with panniers is a long and drawnout affair, believe me)

Good luck, my friend. Your first tour is always the best. Hope it's a blast.

Wow, I;ve got itchy feet, just describing it....