Sub40 10k training (General inquiry)

[MitchelC]

I did my 5k pace run today; aiming to get sub 21:15
Couldn't maintain pace from the very start, right calf flared up again :/ found this one extremely difficult

Here are the splits:
4:35
4:33
5:00
4:53
4:47

Ending up with a time of 23:47

Hoping i could do better next interval (3x1600m)