What is entailed upon me after I said, "As long as I am healthy, I will pray four rak'ahs to Allah after Maghrib, two by two," knowing that I did not intend it as a vow, and should I pray them directly after Maghrib or whenever it is convenient for me, and must I obligate myself to them?

The statement "I will pray to Allah after Maghrib" is not considered a vow (nadhr) that must be fulfilled unless the intention of a vow was present. A vow is established through phrases that denote commitment, such as "It is incumbent upon me for Allah." The mentioned phrase is more akin to a promise, and while performing what is stated therein is religiously desirable, it is not an absolute command, and one who deliberately and voluntarily abstains from it does not incur sin.