Races are run over a variety of different distances. In the UK flat races range from 1km to 3km, UK Steeplechases from 3.2km to 8km and US races from 1.2km to 2.4km. So you can see they vary considerably.

Most horses will have a distance which they give their best performances. Some horses will be very fast, but with not much stamina. So these horses will perform better in shorter sprint races. Others will not be so fast but will have more stamina, so obviously these horses will fair better over longer distances. Looking at a horses performance will help you to work out which distance is the optimum distance for that horse. If has run well over 1 mile(1.6km) you can be fairly sure it will also perform over 7 furlongs (1.4km). If however the furthest a horse is raced is 7 furlongs and it is entering a race of 1 and a half miles then you must be cautious as it may tail off at the end of a race this much longer than it's used to. Often though, if a horse has good basic speed it will be able to win over a number of different distances.

It is useful to keep an eye on distances but it is not the most important factor to consider compared to others. Most of the time trainers will only enter horses in races with a length they believe their horse can perform well.