CLASSIC Ride of 91km, 1050m of ascent and Medium grade. GEARS: Single possible but standard 39/25 or compact; PARKING: free at the start; BIKE SHOPS: Bertie Maffoon's, Marlborough 01672 519119; INFO: Marlborough 01672 513989, Avebury 01672 539425, wiltshirewhitehorses.org.uk. 3000 years ago someone carved a 320m stallion in the Oxfordshire chalk downs, now 24 are known in Britain's hillsides, 13 in Wiltshire - 8 can still be seen today, and here you visit 4.by thunderthighs