This is an interesting letter from Seneca and I've been procrastinating writing about it, because I've had a bit of a hard time pulling out Stoic principals.
The topic is retirement. By this, I suppose he means roughly the same thing that we mean today - to cease, generally speaking, the day-to-day work in which you are employed.
Not that I would advise you to try to win fame by your retirement; one's retirement should neither be paraded nor concealed. Not concealed, I say, for I shall not go so far in urging you as to expect you to condemn all men as mad and then seek out for yourself a hiding-place and oblivion; rather make this your business, that your retirement be not conspicuous, though it should be obvious.
The golden mean applied in retirement too. Don't be extravagant; don't make it oblivious. Find the right approach to retirement when you enter it.
Depending on your personality (extrovert or introvert), you may be saddened to leave all your work acquaintances or you may be relieved. But none of this is good or bad. Rather what is in your attitude is that matters most.
Depending on your lifestyle, saving and investment habits, retirement may bring the same sort of life or it may force you to live below your means. Also, depending on the type of power and authority you had while working, you may have the same or less, when you retire. All of this is indifferent and quite subject to change when you retire. Also, if you've not done the proper philosophical contemplation regarding these indifferents, retirement will force you to finally accept the truth.
We hold that there is a succession of causes, from which fate is woven; similarly, you may be sure, there is a succession in our desires; for one begins where its predecessor ends. You have been thrust into an existence which will never of itself put an end to your wretchedness and your slavery. Withdraw your chafed neck from the yoke; it is better that it should be cut off once for all, than galled for ever. If you retreat to privacy, everything will be on a smaller scale, but you will be satisfied abundantly; in your present condition, however, there is no satisfaction in the plenty which is heaped upon you on all sides. Would you rather be poor and sated, or rich and hungry? Prosperity is not only greedy, but it also lies exposed to the greed of others. And as long as nothing satisfies you, you yourself cannot satisfy others.
Seneca seems to admonish the retiree to embrace the lack of abundant money, power and prestige. While a person may never have done the philosophical work of limiting desire for indifferents while fully employed, perhaps there is hope yet for them in retirement.
That last statement is impactful: "as long as nothing satisfies you, you yourself cannot satisfy others." Those who never limit their desires can never help another person. The game of desire is an un-winnable game. And everyone who plays it, loses. But the person who limits their desires can win (enjoy eudaimonia) and they can be in a position to help others win too.
A worry of many who wish to retire is: "will I have enough?" Seneca poses this question in a slightly different way: "how can I take my leave?" To which he responds with noting that said person, who wishes to retire, already is worrying about their day to day activities. They are worrying about ensuring new ventures make money and all such manners of toil and stress. Why should the stress of not having enough money be any different? You worry when you work; you worry when you retire. It's all worries! He uses a different analogy to express this sentiment. One may worry about being struck by lightening in the valley and and even more so on the peaks of mountains ("There's thunder even on the loftiest peaks.") In retirement, stay in the valley, "hug the shore."
The ending quote of the letter is another one by Epicurus.
You must reflect carefully beforehand with whom you are to eat and drink, rather than what you are to eat and drink. For a dinner of meats without the company of a friend is like the life of a lion or a wolf.
Giving favors to win friends may be productive, but mind the real cost! Instead of thinking about what favors to give people, thing more carefully about to whom you will grant favors - consider their character wisely! Don't become like the man who abounds in fortune, who thinks he has many friends, when in fact they are not friends, but sycophants.