All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and messenger.
You are required to make up for the days that you have missed in Ramadan if you are able to do so. In addition, you have to observe the expiation for having delayed making up for these days if you were able to observe them in their due time and were aware of the Sharee'ah ruling concerning this delay. If you were ignorant about the impermissibility of the delay or were not able to make up for those days in their due time, then no expiation is due upon you.
If a trustworthy doctor tells you that your condition is incurable and that your ability to fast is not expected, then you may just feed a poor person for each day of fasting that you have missed. The amount of food should weigh about 750 gm. The food should be of a type that is commonly consumed in your home town.
However, it seems that you are able to make up for the days that you have missed even if you have to do this gradually, since you are able to fast in Ramadan. Once again, you must make up for the days that you have missed as long as you are able to according to your conditions and capability.
Allaah Knows best.