This is the Basic of all things Fitness and Health that you MUST understand.
When we provide a Physical Stress to our body, it Adapts to the Stress.
Important to understand that this is like Medicine. You need to take the Right Medicine for a Specific Ailment. So, Medicine differs for Headache vs Typhoid vs Yellow Fever. Headache Medicine can therefore be expected to be a cure for Headache and will not for instance be the right Medicine at all for say, Yellow Fever. (Also important that the Right Dosage of Medicine is given).
Same case with Fitness/Exercise Routines too.
Just like each medicine is given for a specific ailment, so too specific Fitness Routines/Exercises are necessary to create specific adaptations for the body.
So, if you must run a marathon, then, Yoga would not get you to do it. Why? Because, the kind of Stress that Yoga provides, will NOT cause your body to adapt to a high Endurance event like Running a Marathon. Yoga will however, get your body to be VERY FLEXIBLE and does have overall health benefits… But, will NOT cause your body to adapt to running a Marathon.
In the same manner, Running a lot of kilometers (slow/medium/fast etc depending upon the various schedules put out by a proper Marathon Training program) will help you run a Marathon….But, will not cause your Overall Strength to go up Significantly.
Understand that all exercises will cause someone who has been totally Sedentary to get healthier. So, performing Yoga will get a very UnHealthy, Sedentary individual to get healthier and thereby, cause him/her to run more than before. However, running a Marathon is a High Endurance event and takes a lot of specific training to cause our bodies to adapt appropriately (and sufficiently) to finish the event successfully. Doing Yoga will NOT make that happen.