JB,
The main Premium Strength plans do not address core much, very lightly. IF you want to really improve core, the Core plans are the way to go.
For the Premium Strength plans, the Level simply matches the duration of the main plan it’s aligned with. The HM Premium Strength plans are all exactly the same, no difference between Level 0 and Level 3.
The General Mobility plans are an exception, the Levels there are really harder from Level 1 to Level 3.
“if I plan to train for a race in the Fall would the longer plan be a better fit vs repeating/extending the shorter plan?” Not sure what you mean here. Can you clarify.
David