Complex sentence: The heavy favorite going into the Kentucky Derby, Johnstown got off to a slow start under jockey James Stout, then he took command by the mile pole and defeated Challedon by eight lengths.
Split into simpler sentences: The heavy favorite going into the Kentucky Derby, Johnstown did not disappoint. Despite a poor start, under jockey James Stout he took command by the mile pole and defeated Challedon by eight lengths.