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Rabeinu Yonah on Avos (via Sefaria):

הוא היה אומר כל שמעשיו מרובין מחכמתו חכמתו מתקיימת. שבזמן שמעשיו של אדם מרובין מחכמתו נמצאת תאותו לחכמה גדולה מחכמתו. ונמצא כי בכל יום ויום מוסיף חכמה על חכמתו:

He would [also] say: Anyone whose actions are more plentiful than his wisdom, his wisdom endures: When a person’s actions are more plentiful than his wisdom, it comes out that his desire for wisdom is greater than his wisdom. And it comes out that he will add wisdom to his wisdom each and every day.

וכל שחכמתו מרובה ממעשיו אין חכמתו מתקיימת. שנמצאת תאותו לחכמה פחותה מחכמתו ונמצא כי חכמתו תהיה פוחתת והולכת. כך פירשו הראשונים ז”ל. אבל יש לשאול איך אפשר להיות מעשיו מרובין מחכמתו ואם אינו יודע התורה והמצות וכי יש לו לעשות המעשים האלו על מה עשאם. אלא שזו המשנה דברה לפי עצה טובה נאה ומקובלת שיש לו למי שאינו יודע שלא יאבד נפשו ושיקבל על עצמו לעשות כל הדברים אשר יגידו לו החכמים ולא יסור מהן ימין ושמאל כאשר ידעם ועל פי התורה אשר יורו אותו ועל המשפט אשר יאמרו לו יעשה ומיד שקבלה זו יקבל על עצמו בלבב שלם ובנפש חפצה מעלה שכר כאלו קבל עליו כל המצות. ועל דרך זו אמרו כל שמעשיו מרובין מחכמתו כי אף מי שאינו יודע ואינו עושה נקראין מעשה על שם כי יש לו שכר עליהם כאלו עשאם מפי הקבלה. וכן מפורש באבות דר’ נתן (כב.) דתנן התם. כל שמעשיו מרובין מחכמתו חכמתו מתקיימת שנאמר נעשה ונשמע. שהקדימו ישראל העשייה לשמיעה והיה להם לומר נשמע ונעשה כי קודם שיוכל לעשות את המעשה צריכין לשמוע מה יעשו אלא שקבלו עליהם תחלה לעשות כל אשר יצוה עליהם וישמעו ומיד קבלו שכר כאלו עשאם. וכל שחכמתו מרובה ממעשיו וכו’ שלא יאמר אשנה הלכה זו ואקיימנה אשנה כל התלמוד ואקיימנו ואם כה יאמר לא תתקיים חכמתו. כי צריך תקון המדות תחלה ואחר כך תתקיים חכמתו:

And anyone whose wisdom is more plentiful than his actions, his wisdom does not endure: As it comes out that his desire for wisdom is less than his wisdom, and it will come out that his wisdom will continually lessen. So did the early scholars, may their memory be blessed, explain. However one should ask, how is it possible for his actions to be greater than his wisdom. If he doesn’t know the Torah and the commandments; when he needs to do these actions, upon what [basis] will he do them? Rather this mishnah was speaking according to a fit, good and accepted advice for the one who doesn’t know – so that he not destroy his soul. [That advice is] that he accept upon himself to do all of the things that the sages tell him to do, and not to veer from them to the right or to the left when he knows them. And he should act according to the Torah that they instruct him and according to the law that they tell him. And immediately when he accepts upon himself this acceptance with a full heart and a desiring soul, he brings [himself] reward, as if he [did] all of the commandments. And according to this approach, they said, “Anyone whose actions are more plentiful than his wisdom,” [as] even for the one who does not know and does not do, they are called actions – since he has reward for them as if he did them, on account of [his] acceptance. And so is it explained in The Fathers According to Rabbi Nathan 22:1, as we learned there, “Anyone whose actions are more plentiful than his wisdom, his wisdom endures, as it is stated (Exodus 24:7), ‘we shall do and we shall understand.'” As Israel had doing precede understanding, [whereas] they should have said, “we will understand and we will do” – as before one can do an action, they need to understand what to do. However they accepted upon themselves first to do all that He would command them and [that] they would understand; and they received reward from it immediately as if they had done them. “And anyone whose wisdom is more plentiful than his actions, etc.” – that he should not say, “I will study this law and [then] I will practice it, I will study the whole Talmud and [then] I will practice it.” If he says like this, his wisdom will not endure – as one needs to perfect the traits first and then his wisdom will endure.

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My rebbi, Rav Schachter shlit”a, explains that a good barometer of yiras Shamayim is that a person says to himself, “I accept upon myself to keep whatever I learn. If I learn something new, then I will keep that halacha” – basically the same thing that Rabbeinu Yonah says here.

For many years, Rav Schachter has spent part of the summer in Tannersville, a small town in upstate New York. He learns during the day on a bench outside his home and people come to sit and learn with him. Rav Schachter learns out loud, and he says over many ideas that relate to the gemaros that he is learning. One summer, he was studying Maseches Menachos and came to the sugya of hilchos mezuza. As he was learning the sugya, he noted that if a person owns a home in which he lives during the year, even if he doesn’t live there the entire year, he needs to have mezuzos on that home for the whole year. For example, let’s say a person lives in New York City and goes up to the mountains for the summer. In his home in New York City, he has eighteen mezuzos, and in the bungalow that he is renting in the mountains he needs eight mezuzos. To make things easier, he might take down eight mezuzos from his home in New York City, bring them to the mountains, and put them up in the bungalow. Then, when the summer ends, he will bring them back. The halacha is that this is not acceptable; since he lives in his home during the year, he needs to have mezuzos there the entire year.

One morning, Rav Schachter mentioned this idea while he was learning. Later, the people at the table noticed that one of the men at the shiur was not there; he had disappeared. He wasn’t there for Mincha, and he wasn’t there that night. No one knew where he went. The next morning at around 11 o’clock, he reappeared in Tannersville. When asked to explain, he said, “Rebbi, what you described in shiur is exactly what I do! I take my mezuzos down and bring them to the mountains. So when I heard during the shiur that one is not allowed to do that, I left right away and went to buy mezuzos to put up in my house in New York.” This man lived this idea of Rav Schachter! He mamesh lived the yesod of Rabbeinu Yona! Rav Schachter enjoys telling that story very much.