I can run low 4:30s in the 1600. I know that it gets harder to improve as the times drop. What's the main difference between a 4:35 and a 4:25 in terms of speedwork? What kind of 400 and 800 repeats should the 4:25 runner be able to maintain, and with what recovery? I can do 65-second 400s, but after four or five it is hard to maintain the same pace. Just trying to see what I need to work towards in order to reach the next level.