Around 40 Shoujo no Isekai Burari Manyuuki

When she was thirty years old, Inori was summoned to a different world as a hero. Perhaps because her clothes were ugly, she was mistaken for a man, and after defeating the demon king she began ruling the country as their hero king. And now, ten years later, she's being inundated with marriage proposals from beautiful girls!? Whatever the circumstances may be, she couldn't marry another girl, so she starting drinking away her sorrows. Along with the alcohol Inori ended up taking a certain medicine, and somehow she turned into a beautiful young girl with silver hair and blue eyes. With those looks there is no way she would be recognized as the hero king, is there? With that in mind, she slipped away from the castle on a journey! She looks like a beautiful young girl, but inside she is the 40-year-old hero king enjoying traveling incognito in this alternate world!

(Source: AlphaPolis, translated)