Hey! You are correct - if you soley do zone 2 runs - you will become more energy efficient on that meaning your long run speed will become a little faster over time while maintaining the same heartbeat.

The tradeoff with zone 2 is that you train with slower strides compared to tempo runs and interval. Therefore you will also get used to that and thus limiting your top speed!

Therefore in a training program it is often suggested (for example in Pfitzinger’s training schedules) to dedicate sufficient time to speed work aswell using a format like:

Monday: rest
Tuesday: speed work (interval)
Wednesday: mid distance zone 2 run
Thursday: recovery run (with little speed work => 10x 100m strides)
Friday: short tempo run
Saterday: rest
Sunday: slow long run.

That way you balance speed and endurance and should see growth at both goals - while maintaining sufficient recovery time.