This tip has helped some: Keep your elbows relaxed.
At any speed, the elbow should close a bit as it hits its high point in front of you, and open a bit as it moves behind your torso.
This opening and closing is much more obvious with sprinters--check any YouTube sprint vids--but it should be happening at any speed, though it will be *very* slight at LSD paces.
The problem is that, at slower paces, it can be pretty easy to lock the elbow and just move the arm as a unit. (This locked-arm problem is often exacerbated if the runner lets the hands flop at the wrist, in a misguided effort to stay relaxed.) But you can't so easily get away with a locked elbow at faster speeds: the one-piece arm can't offset torque, which is absorbed by the torso muscles.
In future years, you'll want to include a few short (and I mean short: 50m or less, with full recoveries), faster reps (on the flat or uphill) once or twice a week during your base phases. But for now, see whether unlocking the elbow helps.
Oh, and make sure you warm up adequately, including a few easy accelerations, before you move into your current repetition/hill workouts.
Good luck. Please let us know how things progress.