Each day calls for a different tool

 While training for a marathon, one cannot expect to gain the required stamina by running every single day without any thought nor by running the same distance at the same pace in each session.

An ideal marathon plan consists of 12- 16 weeks, depending on the experience level of the runner. No single workout in a week is similar to the previous workout. Variety is the key. 

Some days call for long, slow runs to build stamina and some days demand sheer speed. Depending on the run, the feet might require added cushioning for pounding the pavement and might be longing for the feather- weight on  faster days.