I had to get in a bit of interval work today just to remember what it's like to run fast. It's not a big part of my marathon training, but it feels good to move at sub 5K pace for awhile (2 miles). It comes more naturally to me than to run a 20-miler. Since I'm running a 10K in 10 days, it would behoove me to get in some faster running. And it helps the biomechanics at a slower pace. That's a significant benefit.
Now if only my knee would behave! It feels stiff after any run, but doesn't hurt. So on I go...