Nope, you're not doing it right. Powerback Archtype has speed as a minor and not a major. I'm guessing your next move would be to move off of Strength and cap speed since it's already in the mide 30's...so yeah, that's doing it wrong.
Your other majors are agility, confidence and carrying. You would have wanted to train up one of those instead of speed and get it multi-capped instead of speed. You'll sacrifice some big ALG's on those, especially below lvl 21.
Bad strategy..will be an average HB at best.