Don't lose heart. It takes time. In my 30s, I used to barely do 400m of relatively brisk running. And I'd mentally resigned to looking at running as a thing for the young. Until a wise friend told me about the book 'Born to run'. It was in 2009 Nov/Dec. The book is a fantastic & entertaining read, despite based on Scientific facts. So I recommend it to every body, whenever this topic comes up. Keep on gifting it to friends. Thought of giving it to random strangers in park, who I saw struggle with running, but restrained myself. But whenever any stranger in the park asks me about running, I always recommend the book. Okay, I digressed a lot, so let me get back to the point.

I try to remember the following from the book:

1) Easy then speed - A mistake all rookies make is they go for speed. A thumb rule for easy is, you should be able to breathe easily i.e. easy enough to have a conversation with a co-runner. Not easy, I keep lapsing into running fast, but time and again remind myself of this.

2) Be upright: In your comment you mention you lean forward. Thats a wrong posture. The weight should pass vertically through your body, right uptill the foot. As somebody above said, even the strikes should be below your body. Not too much forward. Head should be straight.

3) Barefoot: I am not fanatic about this. My wife runs with shoes and she does 30km every week, and she is fine with it. But before starting (in 2009) I did use to get some knee pain, perhaps because of trying to run fast. Perhaps it was longer stride (like teenagers run for short distances). Or perhaps it was just lack of belief & practice. Or perhaps it was the heel-strike. Once an old gentleman in the park, I'd developed friendship with, commented that I was heel striking. And I thought, what the heck, just some random old school & out-dated thought. My bad.

But anyway long story short, the book convinced me to run barefoot. And so I started, first with shoes itself, but very consciously toe striking, and it was quite difficult. I could definitely go upto 2-3 km, but my calf muscles used to pain like hell. But knees were perfectly fine. So it happened what the book predicted, and I was encouraged.

And I have been persistent with it. Fast forward to today. I do 41km per week (13, 13 & 15-17 on alternate days). Use a vibram for muddy trails. On concrete do bare foot.

Also I encouraged my wife to start running and after initial resistance, she started in 2013. Then she could barely do 200meters. But now as I said above, she is also a serious (albeit in amateur/recreational category) runner. Her pulse rate dropped from being 80/90 to now in the 60s.

So if you want to, keep at it, and it will happen.

edit: some typos