Is what I intended—to fast on Mondays and Thursdays throughout my life as an act of drawing closer to Allah—considered a vow, and what should I do if it is, especially when there are excuses that prevent me from fasting, such as pregnancy or intense heat?

A vow is not established by intention alone; rather, its establishment requires an utterance indicating commitment, such as: "For Allah is upon me," or "Upon me for Allah is that I give such-and-much in charity." What was mentioned in the question is merely an intention and is not considered a vow.