The Easiest Way to Keep Getting Faster

Have a few "indicator" tests spread out over the week where you measure speed over the same exact distance (uphill climbs are good for this).  Once you can't beat your last time, any more work you do is not helpful.  Go very easy or don't train at all until you're more rested and feel like you're ready to attack and beat the challenge again.   Plan your mileage and easy/hard workouts around that schedule and you'll always improve.