What does strategy execution have to do with caring for horses?

If you were caring for a horse, and had to clean the horse manure, which side of the horse do you need to focus on?

Both of course.

What the horse eats determines what comes out the other end. Therefore, if shoveling seems like a lot of work, check to see what you are feeding the horse.

In strategy execution, go as much upstream as you possibly can. Most initial project definitions address downstream problems, these are really symptoms created by decisions taken way up in the process. Sometimes there are real limitations to how much upstream you can go. Keep trying anyway. Its not a perfect world and that is why big, hairy projects are always done in phases.