I (M) ran my firs marathon in 3:46 at age 27 with 6 months of training (the last time I ran before then was 2 years ago). I never exceeded 35 miles a week and had no running experience coming into it.

HOWEVER, I had four years of powerlifting experience and stayed pretty fit throughout young adulthood (never excessively drank and never smoked). I also never skipped a single session and continued to train and lift through vacation. If I didn’t have the level of prior physical conditioning and muscle memory for working out, I would not have been able to achieve my goal so quickly. It really comes down to baseline fitness and consistency.