You can begin learning Thai before you arrive, and that is a good idea. But studying Thai in Thailand has a special advantage: every day becomes practice.
The classroom gives you structure. The street gives you feedback.
You learn a phrase in class, then use it at lunch. You study numbers, then hear them at a market. You practice directions, then use them in a taxi. You learn polite particles, then notice how Thai people soften their speech. You learn food vocabulary, then suddenly the menu becomes less mysterious.
This cycle makes learning more meaningful.
For future retirees, it also reveals the reality of local life. You learn what you enjoy, what frustrates you, what you can adapt to, and where you still need support.