"March" is short for "[a day's march][1]," which is the distance that can be marched by a group, usually soldiers, in a single day. This is Kipling, after all, an Englishman, so it's not some other archaic or obscure meaning but this meaning, the very English meaning. Exactly how far is that? Well, it's never been set in stone, but the low-end is 10 miles, the high end is 20 miles, and the average appears to be about 15 miles, while Kipling himself, as far as I know, never gave his estimation or opinion. 

Therefore, "20 marches" would indicate a distance of 200 to 400 miles, but probably around 300 miles. That, however, is not consistent with the actual distance from Chini to Simla, which is about 1,300 miles. No one can march 65 miles a day. No one. And certainly not over that terrain, the terrain between Chini and Simla. Apparently, Kipling didn't have a solid grasp of how far it actually is from Chini to Simla when he wrote that bit of fiction.


  [1]: https://www.collinsdictionary.com/us/dictionary/english/a-days-march#:~:text=A%20day%27s%20march%20is%20the,can%20march%20in%20one%20day.