Funnily enough we commented on our group walk on Sunday about a fell runner on Dartmoor. A busted ankle at sunset and he could have been lying there for several hours, cold no food very little to drink. It's not like modern kit heavy.
Fellrunning is a real compromise. Somewhat like alpine climbing. Too little kit and you could be in for a hard time if things go wrong, too much kit and things are more likely to go wrong.
Personally, for a couple of hours running on Dartmoor I carry water, hat, a 2.5 layer waterproof shell (less than 200g), a couple of snacks, e-lite torch, phone, whistle, foil survival bag, lifesaving first aid kit, map and compass. That fits in a bumbag or vest.
For something in mountains, eg the Lakes, I carry that plus more water and food, a Paramo Torres Lightweight gilet, better torch, personal first aid kit and a 2-man shelter. Need a vest for that lot.
If leading a group I'd swap the vest for a rucksack and carry a group shelter, group first aid kit and lots of spare food.