Mile Repeats on the Treadmill

3x 1 mile repeats at 5k pace for a total of 5 miles. Since my most recent 5k pace was about 8:30 and I felt like I could have gone faster, I did the mile repeats at 8:20. The last 1/4 mile of the final repeat I did at 8:00. I felt good the whole time. I just felt like I was running hard which is a good thing and I think it is the thing that I am afraid of. Why is it scary to simply run hard? Why do I feel like running should be as easy as walking? The treadmill was set at 1% grade as usual which converts to 8:18 (trinewbies.com).