Its also high, tough and cold, so be prepared.
Many tour operators provide a horse for each person to ride.
Horses are also used for carrying all the equipment. If you are not confident on horseback then there are usually some 'trekking only' tours. There are only a few trekking operators in Cusco offering this trek on a regular basis. Joining a group works out a lot cheaper but it can be a bit of a haphazard affair as far as quality, group size and departure dates are concerned. A company will often say it will leave in two days time and then cancel at the last minute if it can't find enough other people for the group.