Well. It depends. Unarmored, unburdened "hero" athletes can manage to run 100+ kilometers in a single day. The marathon record is just over 2 hours - that's 13 miles per hour! But that's not easy. And trooping along with a 75-lb pack (I love mixing English with Metric) is not quite the same. BUT - a soldier carrying a pack should still be able go 40 miles (65km) a day. But you know what? Making camp, cooking food - FINDING water - really takes time out of the day - so the practical limit over a long haul is more like 30-35 miles ( 45 - 55 km) per day. With a horse or wagon to haul supplies, you can manage more like 60 - 80 km per day in good terrain. Rain, snow, heat and cold can limit the advance rates substantially.
My military gaming handbook (Numbers, Predictions and War by Trevor N Dupuy) gives historical sustained advance rates for infantry against no resistance in favorable weather and terrain as 24 km per day and cavalry as 40 km per day. Due to obvious logistical limitations, armies move slower than just adventurers. So, I think double those rates 48 km per day on foot and 80 km per day with horses seems perfectly reasonable as maximum practical rates. Typically, though, poor terrain, the need to make camp, bad weather, etc causes me to limit my adventurerers to somewhere between 20 and 40 km per day to avoid wearing out the horses, themselves, wagons, and give plenty of time to scout a suitable place to make camp and so forth. If they are using a guarded road with inns, then they should get maximum rates instead. Magic (create water, create food) can make certain limitations go away and allow maximum speed as well. I do allow adventurers to "push" the limits but I raise the risk of equipment problems, injuries and illnesses and making wrong turns and getting lost. NOTE: I always reserve a portion of "random" encounters to be mundane equipment problems, taking the wrong path, having an accident, etc.
Robin
