Overcoming 'marathon shuffle'

I've been running for a few years now, treadmill, parkruns, a 5k, a 10k, all pretty slowly compared to other runners I see. I've made some improvements in my speed, but my pace is somewhere between 11 to 12 minute miles. I'm late forties, a little overweight, short legs, and can only dream of the sort of pace that my running apps think I should be able to run at. I always seem to be at the upper end of my target heart rate.... pretty puffed out really!

I had a year out with plantar fasciitis, then retrained myself to land forefoot. I've been back running for about a year, without too much problem.

I knew my form was not like the 'good' runners. I was trying to increase my cadence, but it seemed counter-intuitive to switch to tiny little steps just to hit a steps-per-minute target.

THEN......

While searching online, I came across the term 'marathon shuffle' and knew straight away that this was me. Very low steps, with hardly any air beneath them. My ankle barely kicking out behind and my knees barely lifting. I found some videos suggesting I work on 'knee drive'. Pushing the knee forward more (not lifting as such) or flicking the ankle up, to get more 'glide', more lift, but forward not vertical.

I've done a couple of treadmill runs since then - both slower than my usual pace, and just having bursts of my new 'knee drive'. Well, wow - my legs are really tired, which never normally happens. My legs can go for miles, even when my lungs are begging for mercy.

So, am I doing the right thing? Should I persist in these bursts - will it ever feel more natural and sustainable? Right now, my legs just feel like lead. They're so heavy to lift. Has anyone successfully moved on from being a shuffler to a gliding gazelle???