What's the best way to tour Europe by bike?

I can't comment on Italy or bike hire prices, but I can advise that: 1. Panniers will be an extremely worthwhile investment. You really don't want to be carrying a pack for a long bike tour (except maybe something minimal with a hydration bladder in it). 2. I think your aerobic fitness will stand you in good stead. If you're not used to biking, your rear end will probably be your weak point (but you'll get used to it after a few days). Make sure you have a decent saddle!

I could recommend the Danube Bike way https://en.wikipedia.org/wiki/Donauradweg. I just know the Part from Passau to Vienna but that some very nice landscape to travel through.

that's a great plan. pont has the practical points down -- although your muscles may need to adjust too for the first few days. After getting adjusted, 75+k per day in not too hilly terrain should be very feasible. My guess is that renting a bike for such a long time may work out as expensive. Maybe look for a good used one and sell it after the trip? Will you be camping? You'll need panniers anyway, but if you have to carry tent, sleeping bag and cookware, you'll likely need 4 and a front rack. Don't go too far north if you camp, April can still be quite chilly north of the Alps. Italy would be great that time of year.

Build in rest days into your schedule. My ideal touring had three days of consecutive travel, usually about 100km per day, with a day of hanging out, with usually a shorter ride. In terms of panniers, get the smallest that will fit your gear. Buying too big a bag means you will almost certainly overpack. Having a few bungees will mean you can fit extra stuff as needed outside the bags on the rear rack.

There's a lot of great info on the touring forum at bikeforums.net http://www.bikeforums.net/forumdisplay.php/47-Touring Sounds like an awesome trip!

Check out http://www.bikely.com for route ideas (http://www.bikely.com/listpaths). Switzerland and France could be really nice as well.

Whether you rent or buy, be sure to put the effort into making sure the bike fits you well and is adjusted properly. Your general fitness will do you well but you will be using some muscles and joints in slightly different ways for long periods of time, and something like having the saddle 2cm too low can make you sore. You can make adjustments along the way, but you want to have it mostly right to begin with and keep those ongoing adjustments small. As others have said, don't wear the pack, at the very least get a rack and devise some method of attaching the pack to that, and bring extra bungees and cords to attach various stuff as needed. Try to build some flexibility into your route so you're not pushing to keep on schedule at the expense of having fun and checking out cool stuff along the way, or taking it easy if you get tired. Read the bikeforums link. Have fun.

Ireland! My boyfriend and I cycled the West Coast of Ireland this past June and it was AWESOME. MeMail if you're interested and I can send you the details.

I'd spend the last month before the vacation getting to wherever possible on the bike and I'd take the saddle I used with me to Italy (which is a destination I heartily endorse).