To be super reductionist here, there are basically two ways of approaching this: top-down and/or bottom-up.
For an athlete that has the endurance but not the speed, I’d use a top-down approach where all variables are staying fixed except for SPEED. An example session here might be:
Week 1- 3x 1-mile repeats @ 11:00/mile with a 35# ruck
Week 2- 3x 1-mile repeats @ 10:45/mile with a 35# ruck
You’d continue that progression until you hit whatever your goal pace was, then increase the weight or interval duration and repeat that process
For an athlete that has the speed but not the endurance, I’d use a bottom-up approach where all variables are staying fixed except for DURATION. An example session here might be:
Week 1- 3x 800m repeats @ 10:00/mile with a 35# ruck
Week 2- 3x 900m repeats @ 10:00/mile with a 35# ruck
Same rules as above in terms of the progression…once that athlete can hold their goal pace for the appropriate duration, you’d increase weight and repeat the process.
The next layer of this would be the use of autoregulation to drive the progression. In the case of this type of endurance training, I let the athlete’s results determine the following session’s prescription. For example, if my Top-Down athlete from above has a tough time maintaining speed once we get to a certain pace, I’ll just hold that constant for a week or two until he’s able to push through that plateau.
I wouldn’t worry too much about traditional endurance metrics (V02, AT, LT, etc.) with a ruck on. In my experience, the best approach is to go by feel, minimize the number of variables, and let the athlete dictate what the progression looks like.