I felt pretty good, so I doubled back a little on the 1.5 mile run around my house, to make it a full 2 miles. This may not sound like much, but the extra half mile gave me about another 300 feet of climbing (my property is really pretty steep). So I got a total of about 7oo feet of climbing in that 2 miles. On the first big climb, my lungs hurt a bit. They hurt on the second climb, too, but not as much. On the last climb - 200 feet over 1/4 mile, my lungs didn't hurt. It felt like my body was being truly efficient in using the oxygen my lungs were pulling in, and that was a very nice feeling.
The moral of the story is - pain temporary, and the more you endure, the better you will feel later.
Happy running.