How do I improve my running stamina?

I'm 49 yrs old and I gained about 40 lbs over the last couple of years. I started exercising about 3 months ago. So far I've only lost about 18 lbs. I have been trying to get back into running. I have completed two 10Ks but I have walked more than run. My last time was 1hr. & 22 min. I can't seem to run more than a mile without walking and my heart rate is still too high when I exercise. I'm still 220 which is about 40 lbs overweight. When I started I could run almost 1/2 a mile and could walk 3 to 5.