I've only gotten serious about running recently so maybe this is a dumb question, but I used to run in high school and I recall during my harder runs my lungs would feel like they were going to burst.
This morning I did 3 miles on a treadmill in about 18:50, which is a pretty quick pace for me and my legs were very tired and in pain at the end of it even though I was not actually breathing very hard. What does this say about where I should focus my training to get better? Thanks