If you are a hobby jogger go ahead and donate. If you are a serious athlete you should not donate unless you are not going to compete or seriously train for a couple of months. I pint of blood is about 12% of your total blood volume. You will definitely notice that running is more difficult for about at least a week after you donate. However, it takes 6-8 weeks before your blood volume is completely back to normal. This will affect your training for quite a while. I've donated many gallons of blood so I'm talking from experience.