People can be so thoughtless in their well meaning efforts. In sickness and in grief we often just don’t realize the distressing effect our words can have. In most cases the less we say the more comfort we can give. Being a good listener is a talent and a valuable gift to give to others. There is an old, very old, book called PET, Parent Effectiveness Training that I still recommend when I am addressing counseling skills.
Now to answer your question of how do you respond to these inquiries? Answer honestly and with as little drama and length as possible. “Thank you for inquiring. I am doing well” and move on to another topic. If you are not doing well, or feeling particularly well when asked you do not owe anyone an explanation. “It has been a challenging journey that I really don’t want to talk about.” is a short, honest reply.
Somewhere along the line of learning conversation skills a lot of us got the idea that we have to answer questions put to us even when we don’t want too. That it is impolite to not explain ourselves and/or give out information asked by others. Actually that is a misnomer. We do not have to give others explanations, information, details, reasons, or even apologies and excuses simply because they ask. We can be polite and courteous without giving information that we are uncomfortable giving.
Family and people you have a close relationship with call for a slightly different approach. Their concern may be more than friendly curiosity and social exchange. It will probably be genuine concern. With family I would start by telling them what you have told me. How hard it is for you to hear how good you look or having to explain in detail what you are experiencing. Be open and honest. Then, explain to them how you want them to interact with you.
Feeling ill is exhausting. Taking care of other people's feelings during illness is just to much for anyone. Take a look at A TIME TO LIVE, Living with a Life Threatening Illness. It will have many ideas for your self care during this challenging time.