Should your aerobic system be underdeveloped, it will take a few weeks for your body to get used to the lower intensity training.
Don’t worry, the body is very adaptable and improvements will happen.
Initially, it will be hard on the ego to go so slow, but if you have a little patience, you will be very happy with the results.
I have found that after taking a full months break after the racing season ends that I need to walk on the uphills when I start running again.
Every season, however, the time before I can run the hills becomes shorter.
Traditionally, it was thought that only anaerobic training (speed work) builds speed.
However, developing the aerobic system first, before attempting any hard work, is ideal; and you get faster without the “wear and tear” (and possible injuries) that often accompanies anaerobic training.