why does my engine do this???

when my engine is warm i'll stop at a store for something shut my engine of for 10-15 minutes. when i get back in the car i go to start it and i have to crank it over for 30+ seconds before it will fire does anyone know why this happens and what i can do to fix it? i drive a 73 scamp with a 225 slant six with an automatic tranny.

thanks
rob