1. If you say it in a joking tone, you can reply like this: "Haha, you misunderstood. I'm not secretly working out. I'm afraid you think I'm getting old too fast. I'm trying to find a way. "
If you say it in a serious tone, you can reply: "I'm not secretly keeping fit, I just want to stay healthy so that I can spend a better time with you." I don't want to be a burden to you. I hope we can grow old together and share the beauty of life together. "