That's the point...you got injured (Achilles).
There is a top high school coach who has sent kids on to college who have had great success. He claims to be a low mileage program. He explained to me that he makes sure his kids run multiple paces in practice, as this also prevents injuries, unlike in the 1970s when many doing LSD 120 mpw were getting injured.
If you run the same speed you are emphasizing how the muscles work in the same fashion over and over again. Putting in some slower and faster stuff will give the muscles a rest because you are emphasizing muscles in a different way.
I prefer not to use the term, "you are using different muscles when you run fast," because that's not really true. Perhaps my quads and hammies come more into play when I am sprinting as opposed to jogging, but I always use all of my muscles when running!
This is also a plug for Igloi/Schul training systems if you want to find out what that is.
There is a danger in doing hard interval training all year long unless you have recovery. The best way to recover seems to be long slow runs the day after the hard efforts.
The OP mentions 6 X 120 as being "very fast reps in 14 secs." That is speed endurance not speed development. Speed development should be done one day per week all year long because that is also an important energy system. Most distance runners want to ignore that one because they are not good at it.
Speed development would be doing something like 6 X 60, skipping rope, plyometrics. It could be broken 200s where you emphasize acceleration, classic running form, and the mental aspects of running. Talk to good sprint coaches about this. Like any other training you must build up to it and have a recovery day!!!
However the long run puts the tiger in the cat because if it is long enough it will stress all three systems pretty well even the first system of speed development. This is supported by research.
You can run 20 mpw but that's not the best way to do it in the long term.