At about 28, your endogenous testosterone starts dropping off. To that end it's harder to maintain your body composition than before. It's not so much that your body slows (it need not), but that the hormones needed to preserve the younger you are no longer present. If your diet is good (you're eating enough of the right things); you're doing resistance exercise; you're doing some level of cardio activity; and getting rested, and still getting fatter, almost certainly you've entered andropause and are a good candidate for HRT (Hormone Replacement Therapy).
Getting on some testosterone, or one of its derivatives (AAS), will preserve your body composition, protect you from diseases of aging, protect your heart, protect your bones and muscle mass, preserve your blood pressure and cholesterol, protect against prostate cancer, and promote a general sense of well being, while allowing you to have higher physical performance well into middle age, and beyond.
Don't forget that maintaining lean muscle mass is the ONLY way to increase your base metabolism, and strengthen your bones, AS YOU GROW OLDER.
In my view, you're much better off to preserve your wellness rather than to treat illness.
See a doctor. Get a CMP, fasting glucose, free testosterone, and free thyroid tests done, along with a PSA, and the usual BP checks, etc., just to see how the human machine is running. Likely, it's time for you to begin more wellness promotion before you end up doing disease treatment. You're almost certainly a good candidate for HRT.
Be careful not to get caught up in Big Pharm's cholesterol drugs, and bp, drugs, that force your levels to levels that can't be maintained without constant medication and side effects.
If you're serious about building fat burning muscle, work your legs. 2/3 of your lean mass is in your legs.
Visible abs are just a function of body fat levels. Even the most out of shape person can have "abs" if their fat level is low enough.