It’s essential to never try to run through more than a moderate pain in a muscle, bone, or joint. Toughness and determination are good qualities, but disrespecting pain altogether will lead to serious injuries that may take a long time to fix.
Dizziness, light-headedness, confusion and blurred vision are symptoms of heat illness and severe dehydration. It’s your body telling you (not asking anymore) to stop and recover.
Persistent fatigue, declining performance, lasting muscle soreness, and low motivation are signs of overtraining, and the only right thing to do in those circumstances is to take a break. It may be a day or may be more than that.
It’s now well understood that you don’t slow down because your muscles have reached their limit, but because your subconscious mind believe that you should slow down because it is worried that your muscles will be damaged soon. Anything that makes your mind believe that limit is not near will allow you to go longer. Self suggestion, reevaluation of the circumstances, new mental models can change that. I like to remember at the beginning of every run about how good I feel at the end of every run, regardless of how bad I felt in the beginning or during it.