It might pay to do some riding before you go to Europe, if only to gauge your fitness. Cycling over flat ground is pretty easy. Most people of reasonable fitness can average 10-15mph (depending on load). So 15-30 miles of travel might only take an hour or two of riding. I don't think you want riding to turn into a grueling experience, but you might find that you can comfortably cover more ground, and see more sights, by riding a bit further each day...