At 59 I would suggest that any high level exercise should be avoided as, regardless of how fit you were 3 years ago, you have had a massive shock with chemo and radiotherapy, your body has aged the time plus an addition for treatment and, as you say yourself, things have slipped.
You could contact a trainer who has experience in people of your circumstance, or perhaps look at your diet and calorie intake to gently reduce this and at the same time gently increase exercise. Even a bit of gardening uses a good bit of energy and flexes muscles; have you considered an allotment?
Another option is swimming if you have a decent local pool.
Personally I dislike cross trainers and prefer to walk the dog. A couple of miles a day helps me.