Nepal is full of ups and downs and Zig-zag paths. So, it is more appropriate to think in terms of hours spent walking, rather than miles covered. This is what we use as a grading system. A typical day's walk last from 5 to 7 hours and involves a number of ascents and descents.
Easy: Up to 5 days walking in total. Usually. 3 to 4 hrs walking a day, with the occasional steep path. You can experience plenty of cultural interest and interaction with local people.
Moderate: More remote country and 5 to 6 hrs walking. Relatively low altitude treks; crossing passes of no more than 4300m. Not more than 5 days consecutive walking.
Strenuous: Up to 12 to 15 days consecutive walking through wild country including high passes (sometimes snow covered) between 3660m and 4900m. Sometimes 7 or 8 hrs trek.
Very Strenuous: Challenging, long distance treks for the fit enthusiast. Occasionally more than 8 hrs consecutive walking, but all treks have rest days interspersed. Each trek crosses at least one pass of over 4900m. Often includes very rarely visited areas.