Is what I said about fasting Mondays and Thursdays for life, as a token of gratitude to God, considered a vow? And what is incumbent upon me if I am unable to adhere to it?

A vow (nadhr) is a verbal commitment to perform an act of obedience to Allah, the Almighty, and it is the obligation of a legally competent and willing person to perform an act of worship. Legally, it is a promise of good by undertaking an act of devotion. These definitions indicate that a vow involves undertaking an act of devotion that is not otherwise obligatory. The specific wording used for a vow does not require an intention (niyyah) due to the strength of its meaning; it is established by its formulation, not by one's mere will. The questioner combining the word "vow" with the intention of "gratitude" does not negate it being a vow, and therefore, he must fulfill his vow. If he is unable to do so, he must offer an expiation for an oath (kaffarat yamin) to release himself from it.