Qada Fast

Situation.
There's a man leaving the Ramadan fasting period of 15 consecutive days due to illness. After recovering, he wanted to replace his fast. He meditated and said, "Because I leave fasting 15 days in a row so I have to alternate fasting 15 days in a row too! Do necessarily so?

Answer.
Not necessarily. No texts are set so, so do qada' fasts according to ability.