More loving your fate in this passage (discipline of desire). Marcus counsels himself that some things, such as your weight or height, are out of his control. His weight and height do not cause him anxiety. How many years he will live - that too he has little control over. So if he is not worried about his weight or height, she should not worry about how many years or months or days he has left to live.
A note about weight. Some will say a person's weight is in their control. Indeed, a person can manipulate their weight up or down. For me, I can gain pounds so quickly over so little food, it's shocking. Other people can eat 13 spare ribs and even junk food and not put on an ounce. I've even known a couple of guys who are sad they can never put on pounds. So, even though weight can be manipulated, there are still limits. Eventually you die of some ailment if you put on too much weight (so there is an upper limit you cannot control) and eventually you die if you weigh too little (so there is a lower limit you cannot control). I think weight was more like height in Marcus' day - it largely did not fluctuate and there was not much control over it.
My point: don't focus on the argument that someone can change their weight, rather, focus on the larger point that Marcus didn't fret over his weight so he should not fret over how many years he has left in life. Don't lose sight of the forest by only looking at a tree.