age 22 after college, trained for a marathon.
Long run in college was 13 or 14 miles, so building up to 20 milers was the difference.
Next jump was around 25-26. I did more miles with doubles and overall pace was a little faster. I ran PRs in all distances from 5k to marathon. I will warn you that about a month after the marathon I was injured for almost a full year. Looking back the training worked, but I should have spent some time doing some exercises to strengthen hips and core. I don't think it would have helped me run faster, but I do think it would have helped avoid injury.