If you do not have time to do things right, when will you have time to do it again. Lack of planning usually leads to not doing things right.
Planning is time-consuming. However, not planning can be the disaster or usually ends up with duplication of effort and more time consuming.
Execution is good but without planning, you can end up executing the wrong things or things that are least important or rank lesser in your priorities. Thus 
planning and execution go hand in hand. They are like two wings of the bird!