Ossan Boukensha Kane no Zenkou

Kane, a middle-aged adventurer, found a rare "Resurrection Fruit" while gathering herbs. He thought about selling it, but then he met a girl seeking medicine to save her companion's life. She looked like Altena, his late childhood friend, so he gave it to her for pocket change.

The girl was actually Anastrea, the strongest adventurer known as the Sword Princess. The next day, Kane suddenly got an invitation to join an S rank party, miraculously defeated the most evil monster and gained lots of money, and was suddenly confessed to by a saint 20 years younger than him! Kane's simple life will change completely because of his encounter with Anastrea.

(Source: Shousetsuka ni Narou, translated)