The most serious i have done is not that serious, but one time i was running a Marathon in our capital and suddenly after 24 km my knee felt strange. A marathon is 41,195 km. Like my kneecap was on its way out of its place. So i had to stumble on one leg the rest of the route. Maybe the last 5 kms or so my knee felt a little better so i could begin walking normally again. But it was a long distance to stumble, it was in a city so i was in no danger but i didnt knew the city so i couldnt take any shortcut even if i wanted. My longest run up until that point was 32km but i made it.
Another time was after school i was just a little kid and i had to bike home normally this takes 15 minutes or so, but this day there was a snowstorm and the snow had stockpiled on the road cause it was a small rural road. The snow reached up till my waist. And i had to drag my bike through this aswell. It was really tough. At one point i didnt have more energy in me so i just laid down in the snow and gathered some energy. When i had energy to move again i said fuck this i go up on the field where there not much snow. But i had to climb the small hill to get there not much maybe 1m or 1,5meter but with my bike and through snow it was hard but i made it. Then later the road split in two and i thought i would turn left whereas normally i would just go straight, because turning left gave me some cover from the wind. But it was a mistake because here again i ran into an area where the snow had piled up. So i had to go through this deep snow again. The trip took maybe an hour or a little more and when i came home i had nothing left in me and just dropped to the toilet floor.