Help! I've become a heel-striker

walk on 'em

My first thought was strides. But, seriously, if you aren't in pain from some heel striking then don't worry about it. In your marathon training runs I'm sure you probably start out with your "normal" stride and strike, but as you tire you are naturally going to shift that strike more to the rear of the foot. It's normal. Running 42.195km on your toes is not realistic for 99% of the world's population in my opinion.

I'd cut back on mileage for a bit and incorporate some barefoot running into your training. I was doing too much mileage recently and developed bad achilles tendonitis from heel striking. For the past few days, I've been doing about 20 min of easy barefoot running before I put my shoes on and continue the run, and it seems to be helping. Another thing that helped me was massaging my calf muscles and IT band which were overly tight and hurting my form.

You've noticed from still pictures that you are on your heels?

Even if you are a forefront striker the downward inertia will cause your heel to briefly strike the ground.

Maybe you want to be sure before you try to change anything.

You can't tell from pictures what your feet are doing. The movements are so subtle. They have to be, when you are doing 800+ strides per mile.

Yes! This, too. It can look all the world like you are heel striking in stills when, in fact, you are not.