Orienteering and Fell running?
Most orienteering clubs organise the occasional Long-O. Several hours running round the fells, navigating your way with map and compass and being self sufficient. Usually running solo, but they are low-key, friendly events so pairs are often OK. Usually hot drink and cakes at the end. Low entry fees.
Beer trails and ChocO organised by orienteering and fell-runnning clubs. Follow a cross-country route point to point. Totally self-sufficient, done in your own time. Usually free to club members, and a cache of chocs or drinks at the end. Solo, pairs or very small groups.
Fell Running Club events -mass-start races over a set course, often marshalled.
Big Mountain Marathons/Adventure Races: usually organised by commercial companies so can be very expensive.
One or more days carrying all your own kit. Base camp with marquees, refreshments, camp site etc. Most need a good degree of navigation/fell running experience, but some are geared towards beginners.
This sort of thing
https://theomm.com/events