You might be able to use some world-building books.

Brandiwyn🎶 and I were talking about that last night. Part of it would change due to terrain you're crossing, but an average person walks 4mph. How many hours in a day do you want to walk? How much time would you spend eating and sleeping? I think we put a rough guess on 50 miles per day.

It would also matter how conditioned your characters were. Some of them might cover that distance much quicker. You could also check a marathon page to see how long people covered long distances - and remember that they have been training for those accomplishments for months or years.