If the four English three-thousanders aren't enough for you, here's a much tougher variation that avoids all the long low-level stretches of the classic challenge route, and takes in a lot more hills instead. The main additions are the Dodds, and the long tiring finish along the high ground from Great Gable to Cat Bells. Done within 24 hours it'd be a bit like a poor man's Bob Graham, but it's big and hard enough that most people will need a weekend (or longer) - with a bivvy, a wild camp or even (for softies) a B&B somewhere en route.
46.25 miles, 74.43 km, 4,919m ascent, 28 hours. Keswick