Here is a little more information on these 2 females who are part of my training group. The 16 year old is very much more fast twitch dominated as she is a 58 second 400m runner and at times can finish off track sessions w/2oo efforts in 26-27. The 14 year old has more endurance with 10:06 pb for 3k.
Clarification on Sunday run with easy running but hilly terrain so 16 year old runs 11-12k in roughly an hour but we walk a few steep hills to keep heart low. 14 year doesn't run with us on Sundays. Her volume according to strava is low but she does other sports that have a fitness component.
We only do one track sessions per week all other sessions on grass. (This is the same for my 3:43 1500m male and 1:49 800m male)
Threshold pace is roughly 4min k pace on grass and 5-8 seconds per k faster when on the track. Both girls have pb's of roughly 18:30 for 5k.
For our slower runners we also do threshold efforts over shorter distances but with more efforts.
Our early season work includes short hills and winter season includes runs over hilly circuit around 8-9kilometres.
For the coach with the slower runners I would recommend shortening efforts and slowing things down. To much work at 800-1500m pace for beginners doesn't do a whole lot to improve their aerobic system. When we do run hard it is either at 3k effort (V02) or at 400-800m effort. When doing 400's at 3k pace equal rest when at 1500m pace generally 40 seconds more rest. There are times when doing lactate tolerance work we will have very short rest but longer rest between sets.
Good luck!