Around here (Pacific NW, where we have hills and mountains) I consider a ride with over about 60 feet of elevation gain/mile to start to be hilly, and over 100 feet per mile seriously hilly. Last summer I biked in the Dolomites in Italy, and averaged almost 150 feet/mile. That was pretty hilly... It's hard to do a century up here with only 3200 feet of climbing. If you get a chance to travel to a hilly or mountainous area, if you at all can, find a way to get some riding in--the best way to improve at hills is to ride them. (Full disclosure--I will gladly ride up mountains all day long, but am a whiny wimp when it comes to wind. And when I have traveled to places that have serious wind, I have studiously avoided the opportunity to improve my ability to bike in the wind...)