Here is an example of a fat-adapted older guy (early 60's). He eats around 80-100g carbs/day (as I do) from whole foods, eats lots of healthy saturated fats, and doesn't have to run 50 miles/week to stay low in body fat:
https://www.bulletproofexec.com/wp-content/uploads/2014/03/mark-sisson-inspirational-fitness-photos.jpg.jpg
I look similar to this (around 7-8% bodyfat), and run 30-60 miles week because I love running. I could maintain this body composition on 20 miles/week easily (and do, during "off" periods) without any fluctuation of weight or body composition.
The main problem that older athletes have with this is having the mental flexibility to even comprehend or want to change the fundamentals. Most would rather continue pounding carbs and pounding miles forever, while digging a deep hole of oxidative stress. I see older cohorts struggliing with this, adding pounds around their middle, losing muscle mass, despite putting in the hard efforts of their past.
High-carb is literally a dead-end for most, as evidenced by our obesity/diabetes epidemic of the last 30 years, due to governmental high-carb/low fat recommendations, which were based on faulty, politically-skewed research. "Big Fat Surprise" by Nina Teicholz documents this tragedy of epic proportions.