I mentioned weight training, not by accident. Steady state cardio using the lipolytic energy system[1] is a very slow way to create a caloric deficit; and it causes atrophy. It's not unusual for Olympic marathon runners to have higher body fat percentages than their sprinter compatriots.
I admire your grit, but have you tried some weights or perhaps some high-intensity interval work?
>It's not unusual for Olympic marathon runners to have higher body fat percentages than their sprinter compatriots.
It's not only unusual, it's unheard of! Elite marathoners have far lower levels of body fat than other athletes, with men clocking in at around 3%! Even unusually lean sprinters have about double that.
I think the main reason for the misconception is that lots of people compete in marathon races and some of them are fat. So people see someone who can still run a marathon but is still fat and think, "ha they can run for hours but they're still chubby!"
But nobody really competes in a 100-400m race after school age unless they're elite or semi-elite... and obviously those guys are lean sprinting machines. That's the only kind of person we ever see sprinting. Compared to that zany uncle who decided to start marathoning upon reaching midlife, elite sprinters on TV are manimals!
If we reversed it though, and compared everyone fit enough to complete a 400m race, at any level with elite marathoners, the results would be the opposite. We'd be saying wow... those skinny marathoners can run 400m faster than most racers, and then do it again 104 more times without rest! Woah!!!
I've done weight training in the past and I've tried interval training. I haven't found either of them to be more helpful. I plan to switch back to weight training around March and give that a try for a couple of months to see if I can find a way to make that work better.
Part of the problem is that the more research that's done into exercise, the more we see that what works for one person may not be effective for someone else. A lot depends on your metabolism and the way your body responds to the various hormones that control energy use and storage.
That's well and good, but (1) most folks are not outliers and (2) thermodynamics is not subject to hormones.
As I said above: the "black box" is enormously complex but it is not observable, so it's not useful for an ordinary person. Caloric deficit works for everyone and it can be controlled easily.
I admire your grit, but have you tried some weights or perhaps some high-intensity interval work?
[1] http://www.hussmanfitness.org/html/TLEnergySystems.html discusses the four energy systems at a high level.