"The best thing was the mud, because it was all squishy and gooey and Mrs Walters went surfing down the mud on the hill," says eight-year-old Callum. Along with fellow pupils at St James the Great Primary and Nursery School in East Malling, Kent, he's just finished a four-week health challenge where pupils decided to walk the equivalent of Land's End to John O'Groats between them. That's 875 miles, which Harry,...