Your muscles may feel sore a day or two after a difficult run. This is a good sign that your muscles are developing.
It's okay to walk if you need a chance to get your lungs back or let your legs recover.
When running, you will have to endure mild pain and discomfort, but only to a point. If the pain becomes sharp or you become light-headed, stop immediately. Your body knows when something is wrong and sends pain signals to your brain as a warning sign. Get some rest to alleviate your symptoms. If that doesn't work, consult your doctor.
But try to avoid such situations and don't push yourself to extreme limits during your workouts. It has a negative effect on the training process and might decrease your workout motivation.