For those that aren't familiar, a negative split simply means doing the second half of your run faster than the first half. I am a big fan of the negative split run mainly for the aerobic gains, but also for the challenge.
I ran the first two miles at a pace of 8:15/mile. At the 16:30 mark, I dropped the pace to 7:45/mile. Overall, it equated to a 4 mile run in 32 minutes, an 8:00/mile pace. However, I slow it down to start and speed it up to end and it makes it a stronger run.