It is not permissible to observe a voluntary fast with two intentions, the intention of making up a missed fast and the intention of observing a Sunnah fast.
It is better for the traveller who is travelling a short distance not to fast, but if he fasts that will be good enough. It is better for one who finds it too difficult to fast because of sickness not to fast. If he knows or thinks it most likely that fasting will harm him or lead to his death then he must break his fast, so as to ward off harm. Whatever the case, both the traveller and the one who is sick have to make up the fasts they missed in Ramadaan on other days, but if they fast even though that causes them hardship, that is valid.
And Allaah is the Source of strength.