When you think intervals, the first thing that comes to mind is 100% effort and total exhausion. That shouldn't always be the case, especially when you are not rested or haven't done any recent hard interval sessions.
On Wed this week instead of my usual sizzling 800s, I did 4 x 1 mile in 6:24, 6:19, 6:20, and 6:12 with 2 min recovery. A very good workout that came close to simulating 10K pace, but not all out ( recall that I recently turned in 2 x 2 miles under 6:10/mile pace). That's also a good sign when the last one is the quickest. I had something left.
I'll do a hard interval workout again soon, but not without a real easy day beforehand.