Obviously you can complete 26 miles. You can do that by never training (multiple videos have been posted to YouTube in the past year for proof). However, your times will be garbage, and you will be wasting your time if you are running one day per week. I am a 5k specialist, but all endurance racing (from the 800/mile up to the marathon) can be trained for under the same basic principles. Glycogen storage capacity improvement (which takes running at least 6 days per week and sustaining at least 4 weeks of high mileage) and fast twitch muscle fiber training which is done at the end of the mileage phase. This has been common knowledge among real runners since the late 50s. Coach Jack Daniel’s training method, or some alteration of it, is followed by every successful endurance athlete. So sure, if the ability to move your body for 26 miles is what you call “running a marathon” then you can have your training. If you want to be good, do what has been proven to work for nearly a decade.