Thanks for writing in with your questions. You may want to listen to the book club recording I just did last night on Chapter 6 of TftUA on Strength. We should have it uploaded to YT very soon. You can find it here.
In the meantime, here are my thoughts on your questions:
Hill sprints, as you might recall from pages 179-183, are very short near maximal efforts.
The outside muscular endurance workouts are long-duration sub-maximal efforts.
They can be used concurrently (in the same week) during your training. I’ll be going into more detail on how to implement these modalities in the next book club meeting in late September.
As a rule, general strength comes earlier in the training cycle than hill sprints if one is deficient in general strength. At some point, you’ll replace the general strength with the hill sprints.
I hope this helps,
Scott