By Martyn Herman LONDON, July 28 (Reuters) - Mark Cavendish's past Olympic appearances have not exactly gone to plan but the Briton says he will arrive in Rio on a high as he bids to justify his return to the velodrome boards with a long-awaited first medal. The 31-year-old world road champion will ride the multi-discipline omnium event in Brazil, having convinced the British Cycling selectors he has a genuine medal chance. Since then Cavendish has juggled track training with his 'day job' for Team Dimension Data with whom he won four stages at this month's Tour de France, taking his overall tally to 30, four behind the record of Belgian great Eddy Merckx. More...