From the books set in the very early times of Pern it is clear runners were already in use, since the need to send messages; on a planet that intended to leave technology behind, would have been great, and therefor these would have existed from the very beginning.
The earliest runners are descibed in the books as having been much faster and having more stamina, either the traits they valued were diluted within the gene pool over time, or the whole population of Pern has changed in perhaps size and Physique (this is very likely as they moved to a totally different planet, not only owuld gravity be different but also the environment itself this would have led to quite pronounced changes in the 2000 or so years to the 9th pass! However the traits that runners pride; thin (good long distance runners), to some degree speed, and a strong stamina, would continue to be important and selected for. The books say that runners tend to chose spouses within the craft, as in other runners therefor mainting their traits. Although runners from none runner families could become runners it is less likely as they would simply not be strong enough or have enough stamina to keep up with those runner born.
They do not have craft halls! Instead they have runner stations, each station covers a particular area. They store and sort meassages and keep the runner traces in a good condition. Traces are the specialised tracks that runners use, they are bouncy made from a combination of sand/shaving etc, as they would otherwise hurt the runner's feet. In 'Runner of Pern' it is also stated that only runners are usually allowed to use them as they were special soft footware (have spikes on sole) that are designed not to damage the traces.
Although they have apprentices and journeymen/women, their stationmasters are usually retired runners or those unable to run for whatever reason. 'Crossing' refers to running across the entire continent. There are also small shelters that are maintained by the station where runners can hide from storms and thread.
Runners need a good ability to recognise weather changes, they would not want to be caught out in the rain with no change of clothes, they would also know when and where thread is to shelter in time (would also know where shelters are) clearly they need a good memory. The should have the traces and surrounding areas memorised so that they could run in the dark and know where and wen to find water. Runners also carry basic healing equipment (e.g. numbweed) and would have a basic knowledge of how to use it. They also need to know how to make and repair their specialised clothing and shoes.
Their clothing must be comfortable but cooling and warm! Their tunics are long enough to cover their stomachs and abdomen to avoid unwanted injuries and damage due to cold, their trousers are short (like 3/4 inch trousers) in winter, and shorts in summer. Their shoes are soft and durable (boots in winter) and have specialised spikes on the bottom to grip the traces.
Like all crafts the runner stations vary in size and importance, while there may be many small family run stations with just a few rooms and a few amenities, there may also be huge complexes such as fort is descriped in RoP, where the baths are said to be heated continually (probably put there by origional colonists using technology no longer available).
Runners are used to carry messages for the avarage person who doesn't have or have access to a runnerbeast. The north is already a crisscross of runner traces all over the continent. The South as only just beggining only have a few, but more are made and levelled out by the controlling station all the time.