Well this question is relative to what kind of horse you're talking about. Different horses are trained for different purposes but as far as traveling is concerned then let's consider endurance horses(horses which have been trained and bred for long distance travel), I'd say they could probably max out at 100miles in a day.

Us cavalry horses are an example of endurance horses and they were expected to travel about 100miles a day with around 300lb loads.

I hope this answers your question.