[JustAnotherJarhead]

I like the idea of two different skills sets in running, an olympic sprinter trains very differently than a marathoner, but I don't think it's important enough on game play to make any changes.

But, I'm am wondering, can you really "sprint" in full plate armor, with shield n sword while wearing a full backpack and a wineskin?

I don't think anyone is running anywhere in metal armor, at least not for very long, it's going to be a lesson in conservation of energy to make it through the day.
If a walk is MA 3 or less, then you are "running" in your plate armor at MA 6, you just aren't sprinting like your unarmored team mates at MA 10.

I think running talent RAW in current Legacy is just fine, and to be fair, even allowing elves the faster MA while in 16 lbs of boiled leather armor is probably a stretch, it's stiff, it has weight, it moderately protects.

Even giving Cloth armor the same MA as an unarmored man, is very generous considering it's only 2 lbs lighter than leather, and nearly as stiff.

If an unarmored and unencumbered character can achieve an MA of 10 on a dead sprint (no one says for how long he can sustain that rate), then there is no realistic way an armored character can keep up, even with training.

I think the 6 MA for metal armors is probably fair, and taking note that Steve and crew didn't extend the Eleven running affinity to include metal armors says a lot.