I've been aboard the threshold train for over a decade, but I disagree with those who say race pace workouts don't improve fitness. Your specific biology might change the way your body responds to workouts, but "5k pace workouts don't improve fitness" is incorrect.
The workouts themselves are only one factor. The specific workouts that you're doing might be too hard for you in the context of the other training you are doing. There are several factors you can change:
The total volume of the workout
The pace of the intervals
The rest between intervals
Mileage on easy days
Pace on easy days
Days between workouts
You don't say what specific workouts you're doing. If you're running 6x1k at 5k w/ 1 min rest--yes, that's going to be too hard for a lot of people. Change it to 2x1200 threshold w/ 1 min + 4x1k at 5k w/ 2:30 rest and you'll get a lot of benefit without digging a hole.
Also think about those other factors. Most of the best runners I've coached intuitively slow down their pace when they're tired. For example, I coached a girl who could do long runs under 7 min/mile average but would run close to 10 mins the day after a race or challenging workout. A lot of perpetually burned out runners just do the same pace every day.