I like 'yomp', because the sound of it (and the feel of it if you say it) fit so superbly with the meaning.

Very true. Since I first heard it as a 14 year old watching TV coverage of the Falklands conflict, I have loved the way "yomp" rolls off the tongue, and it is very evocative. That could be largely by association, but the word brings back vivid images, and seems to hint at the exertion involved in yomping.