I went looking for how old ahimsa was before he took it up: centuries older — Jain, Buddhist and Hindu teachers argued over it mostly as a discipline of not injuring any living thing, down to what you ate. He was born in Gujarat in 1869, where that argument was close at hand; his mother kept long fasts. Somebody always spun, somebody cooked. The birthday is now kept in two calendars: India's, and the UN's since 2007.
The Salt March is the bit I always go back to for the logistics of it: about 385 km from Sabarmati to Dandi, 24 days on foot, and roughly 78 who set out from the start and walked the whole thing. Water, food and sleeping arrangements found village by village along the way, in 1930.
The second calendar was fixed on 15 June 2007, when the General Assembly adopted the resolution without a vote — well over a hundred states signed on as co-sponsors, which for that chamber is a quiet sort of unanimity. India moved it; the text asks for observance rather than anything binding.