One 'march' is the distance that can be travelled by marching or walking in one day. It may vary depending on the route and type of terrain.
Rudyard Kipling was a careful writer and professional journalist. Even at the age of 18, when he wrote 'By Word of Mouth', his first published work, he would have taken the trouble to make his story accurate.
I am certain that 'Chini' in Kipling's story is the scenic Himalayan (mountain) village formerly known as "Chini Village", "Chini Gaon", and now officially called Kalpa. It is in the Sutlej River valley in the Kinnaur district of India, some 160 miles north-east of Simla (now Shimla). The area was popular for walking tours. In 1863, two years before Kipling's birth, Samuel Bourne, a Victorian photographer, travelled to India to take scenic photographs.
[He went to] Simla, visiting en route Benares, Agra, Delhi and
Umballa. In Simla he commenced photographic work and on 29 July 1863
left the hill station for a trip into the Himalayas. With a retinue of
30 coolies he travelled to Chini, 160 miles north-east of Simla, and
spent some time photographing in the Chini-Sutlej River area before
heading west to Spiti and returning to Simla on 12 October with 147
Pictures of India (Cambridge University archive)
One tourist map gives a total distance of 255 kilometres, which equals 158.45 miles.
Estimates of the length of a "march" (one day's march) will differ depending on the terrain to be covered, and e.g. whether some the daylight hours are to be used for breaking and making camp, meals, rest, etc. A common estimate used with Roman legions was 10 miles per day.
It is feasible that a distance of 160 miles would be considered a march of 'some' (appoximately) 20 days, especially in a hilly or mountainous region.
This is not one of Bourne's photgraphs!