I'm 55. I'm 5'5". I'm 190#. Down from my 230 of yesteryear, but, I'd submit I'm even more of a stud than I was before, because I have more bases covered. I'm in my 42'nd year of lifting. I do redline cardio at least twice a week. I rank in the top 1% in fitness. I had 3 CABG in 2013. A heterogeneous program for wellness is what I do (But, not for living. For a living I design process and infrastructure, and, enforce that process).
Fitness can be indicated by a number of factors. Statistically, and this is by no means a reach, four things come into play in terms of length and quality of life.
1. Genetics. Some folks live to be 115, never exercising, smoking, and drinking, but never stressing out. Scientists have isolated a number of genes (this is REAL science) in various worm and mouse species that turn the aging process on and off. Famed actor / director Ron Howard most recently did an extensive documentary about this. Genes matter in terms of length, and quality of life. Talk with my cardiologist.
2. Outlook. You must giggle every day. Period. You must now worry, nor awfulize.
3. Exercise. Exercise promotes the same chemicals as giggling, markedly improved vitals, much better wellness, physical ability, cognition, and sleep patterns. Exercise makes a huge difference. PERIOD. It is a must have for EVERYONE.
4. Nutrition. If you eat right, it's nearly impossible to be fat. PERIOD. You don't get to be a 400 pound lard ass without stuffing yourself daily. It's mental illness that needs treating; not acceptance; nor enablement. Fat folks are sick. PERIOD.
Note that the highest incidence of sudden death is among marathon runners. An example of those who damage their cardiovascular system with too MUCH exercise.
To address the question at hand: You can be fitter without being fat. Can you be optimal and a lard ass? Of course, not. Why? Extra tissue lying around doing nothing but lugging you down, and causing you to have higher vitals. Think of fat as a parasite. It rarely give back, and only with a struggle. Sure, you need fat; especially polys and monos, but, being obese is a fool's errand.