Vtyjkyuk

Wanting to keep my writing realistic, I try to keep distances traveled in the humanely possible range.

The current setting is thus:

• Medieval style world, roads available, they are travelling north, climate like Germany/Holland/Scandinavia




• A group of mostly military (only one not-experienced rider among them) horsemen travelling as fast as they can because their life depends on it need to cross a windswept plain. About 12-15 persons plus horses.




• They ride light (light armour) but heavily armed (everyone carries either one or two swords or a bow with full quiver and a shortsword, and an occasional axe or warhammer present).




• They have pack-horses for supplies and enough mounts. Rations are simple dry rations for the men and oats for the horses. Water is available on the plain.




• The not-experienced rider is a thin woman, but she is a magic healer (magic does come at a cost in the form of energy drains).




• They are being followed and they know that. The windswept plain houses many nasties and you don’t want to be there for much longer than you need.

How far could they have traveled in two days and three nights, taking minimal breaks and only one longer rest due to extreme fatigue?

The answer to this depends on the rider's preference: do they want to travel "AS FAST" as they can or do they need to travel "AS FAR" as they can.

Riding a horse in medieval times didn't meant that the rider was constantly on the horse.

Going as fast as you can meant that you could travel 80-100 km with the horse cantering but that also meant that after two hours of constant effort the horse would just drop dead.

Going as far as you can (or going as far as you can in the shortest time) would mean that you are constantly on the move. Riders don't ride the horses all the time but they are walking with them.

A horse is an unusual animal in that after 4 hours of constant moving they will stop. It don't matter how fast or how overladen they were, they just need to rest.

A few examples:

• 
  James Mowat - 350 km in 36 hours. Edit based on info from a Canadian site - Mowat did in fact change horses during his trip and it took him 3 days. (I assume it's because he started late on the first one and finished on third day)


• 
  Ashgabat – Moscow horse ride by Turkish horsemen in 1935 - 4128 km (of which around 900 were through a desert) in 84 days.

Traveling at night, without lights (to not attract nasties) would require riders to walk beside horses. Maximum speed of 4 km/h.

So I would say that 400 kilometres is a safe distance to assume. Maybe 600 if you don't want to keep the horses and can ride them to their death at the end.

Horse distance trivia: Crusaders in Prussia built castles separated by a distance that a rider could travel in one day (from dawn till dusk). As you can see from this picture Crusaders' castles in Prussia the distance was different depending on roads, geography and ease of travel (barbarians, big forests and so forth).

How far can you travel on horseback [...]

traveling as fast as they can

At full speed (gallop): about 2-3 km. Horse gallops at about 40 km/h so that would be about 5 minutes.

Maximizing distance: about 50-60 km per day.

40km is the largest distance that separated Pony Express stations. At the station the rider would change to a fresh horse. A single rider, weighting max 57kg with 9kg of mail, rode about 120 km per day - that's only relevant in showing that such figure is impossible with one horse instead of 5.

Middle Ages horses were not as good, their saddles were not up to par and your riders are much heavier. Wikipedia says about Middle Ages horse riding that "Small mounted companies might travel 30 miles (50km) a day."

Both horses and men needs rest. Riding at night on dangerous plains is simply not practical, the best you can hope for is horse breaking a leg and leaving your stranded there.

https://en.wikipedia.org/wiki/Endurance_riding

Endurance riding is an equestrian sport based on controlled long-distance races. It is one of the international competitions
recognized by the FEI. There are endurance rides worldwide. Endurance
rides can be any distance, though they are rarely over 160 km for a
one-day competition.

There are two main types of long-distance riding, competitive trail
riding and endurance rides. In an endurance ride, discussed in this
article, the winning horse is the first one to cross the finish line
while stopping periodically to pass a veterinary check that deems the
animal in good health and fit to continue. As with human marathon
running, many riders will participate to improve their horse's
personal best performance and consider finishing the distance with a
proper vet completion record to be a "win".

In the United States, most endurance rides are either 50 or 100 miles
(160 km) long. Shorter rides, called Limited Distance rides (LD), are
organized for new riders to the sport or young horses being trained.
However, LD's have evolved into a competition of their own, in which
more experienced riders and horses also participate. There are also
longer, usually multi-day, rides as well. In the USA, the American
Endurance Ride Conference (AERC) sanctions endurance rides. In the UK,
Endurance GB is the governing body. Winning riders can complete
100-mile (160 km) rides in 14 to 15 hours.1

Any breed can compete, but the Arabian generally dominates the top
levels because of the breed's stamina and natural endurance abilities.

Readded as a quote.

SO if 160km in 14 to 15 hours in race conditions I would say the horse wouldn't be able to be used that night and little the next day.

So if you wanted to keep your horses I would say it would be around the 300km mark. If you didn't want to keep the horses then you could probably get this up to 450km but that would mean a hard ride at the end and your riders would arrive very tired.