The English Grand National, not a flat race but it's run in the flat season, making it the longest race in the flat race season.
Indy 500
The Pontefract Marathon Handicap run at Pontefract over 2 miles 5 furlongs and 122 yards.
Many countries. see www.formula1.com for the race course locations
Flat horse racing is when a horse is racing on a flat. a flat is a course without jumps. Flat horse racing is horse racing without jumps.
Surely the grand national - jumps (4M 4F) Possibly the ascot stakes - flat (2M 6F) The longest flat race under Jockey Club Rules is The Newmarket Town plate at 3M 6F. The Grand National is run under National Hunt Rules.
The longest race is the 50 Km walk. The longest running race is the Marathon.
An iditarod is a dogsled race that takes place in Alaska. It is longest dogsled race (over 1100 miles).
The Coca-Cola 600 is currently the longest sanctioned Nascar race.
The Great Serum Race
On a flat course or a racing oval, a greyhound will be the fastest .
If you are referring to competition pools, then your answer is Long Course Meters. This course, used in the Olympics, FINA World Championships, and US Summer Nationals, is 50 meters long. If you are referring to longest swimming competition, it's definitely open water swimming. The longest race held in the Olympics is the 25K.