I'm currently working on distance in preparation for an hm in September, my longest run so far being 15k. Pace is much slower than it was at 5k but it feels like I could keep it up for hours. Am I kidding myself? Could it be the effect of bumbling along in a semi trance like state or does a "comfortable" pace make distance that much more achievable?
In practice, I'm keeping well within the 10% rule because it's more important to me to maintain regular exercise than it is to improve performance.