It is all relative depending on what distance you're racing. If you're focusing on your first marathon or ultra, I'd leave the speedwork focus in favour of increasing your aerobic endurance, maybe incorporating some marathon pace tempo within long runs.
For half marathon/10milers however - tempo runs are great. 25-40mins at race pace - hard, but you need to be able to sustain it for the length of time without breaks. Faster than steady running but not so fast you get lactate buildup and hav e to stop after 10mins. You can also incorporate blocks of tempo into longer steady runs.
10k, 5k, cross country - intervals will make all the difference. Repetitions ranging between 2-5mins generally with short recoveries of 30secs-1min. The recovery should never be quite enough and these are to be run at a hard pace. For 5k and cross country in particular the total reps time will be longer than your race, but you won't have the recoveries in between.