Tempo runs (~85-90% of race pace for 4 miles would be a good start, then work on bringing it up to 5 miles) as well as long hill sprints are great for speed-endurance work. I like at least a 400m long hill for that. By the 400m workout, heck your mile time should be a bit faster. Try longer intervals with short rest (Like 5X1000m or 3Xmile).
Ultimately all of your workouts should progress towards doing more race-paced stuff leading up to your peak race
But realize that there aren't any magic workouts. And also realize that all workouts need to have a sufficient recovery. SLOWLY. Like slower than 6:40 pace for 8 miles. try more like 7-7:15 pace. I ran many less miles, but a much faster equivalent 1500m time.
Also, what is your 5000m race time?