All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
The preponderant opinion in the Shafie's doctrine and the majority of Muslim scholars that the person incapable of fasting as prescribed in killing expiation can't move to feeding the poor; rather, he remains liable for fasting. Further, if he died before doing so, his heirs are free to choose whether to fast on his behalf or to feed sixty poor based on the apparent meaning of the following verse:" It is not for a believer to kill a believer unless (it be) by mistake. He who hath killed a believer by mistake must set free a believing slave, and pay the blood-money to the family of the slain, unless they remit it as a charity. If he (the victim) be of a people hostile unto you, and he is a believer, then (the penance is) to set free a believing slave. And if he cometh of a folk between whom and you there is a covenant, then the blood-money must be paid unto his folk and (also) a believing slave must be set free. And whoso hath not the wherewithal must fast two consecutive months. A penance from Allah. Allah is Knower, Wise. [An-Nisa' 92].
In this regard, Al-Khateeb Ashrbini (May Allah have mercy on his soul) stated: "The expiation of killing has the same order in paying expiations as is the case with the expiation of Dihar regarding Manumitting a slave and if that wasn't possible, then fasting for two consecutive months as mentioned in the above verse. However, according to the apparent meaning of the verse, feeding the poor isn't part of the killing expiation even if there was hardship in offering the prescribed fasting, since, what determine expiations and their order, are the texts of Quran and Prophetic Sunnah nor Qiyas (Analogical reasoning). Besides, Allah, The Exalted Prescribed the killing expiation by manumitting a slave or fasting for two consecutive months and didn't Mentioned feeding the poor at all so long as the liable person was alive; thus, if he died before performing the due fasting, then the amount of feeding one poor for each day should be extracted out of his inheritance as is the case if he missed the obligatory fasting days of Ramadan and passed away before fulfilling them." [Mughni Al-Muhtaj vol. 5 pp. 377].
In conclusion, what have you (Questioner) mentioned in your case that fasting might cause you great deal of hardship permits you to delay it till you gain the required strength or stamina that help you to perform what is due. Besides, you are obliged to notify your heirs in case any kind harm was inflicted upon you to pay the aforementioned expiation on your behalf and we ask Allah to bestow you with a sustainable health. And Allah Knows Best.