Intervals are great to get faster. Remember that there are different types of intervals. short and intense with short to long breaks in between or longer not quite as intense intervals with a bit longer recover in between. You can also do hill intervals which I always felt were very helpful. Same sort of idea short or longer, if you cannot recover in between by going slower up hill, turn around coast down, turn around and go again! Obviously, these are best done on a non-trafficked road.

Also, intervals take more recovery the day after than your normal same speed workout. You may be more tired and you may just need to go slower the next day or take a rest day off entirely. Intervals build your muscles and your cardiovascular output so don't be surprised to be tired after doing them.

Make sure you slowly start with intervals, i.e. do two intervals and two rest periods in your first try. take a few days off and then the next time do three intervals and three rest periods. Mix it up. do short intervals one day; three days later do longer intervals. Change the rest period in between intervals; but most of the time you want to be almost fully recovered so that you can put your entire effort into the next interval. Soft pedal between intervals and get recovered.

Variety is the key. If you do the same thing over and over, your speed will stay the same. If you want to increase speed, increase your variety.