Difficult but you have to be honest and say you are no longer able to keep this arrangement up. If she is unhappy about your decision, suggest you come round/she comes to yours/just for coffee and cake.
Age UK.libraries, CB might have some notions of where you might find a lunch club ....some can even arrange transport to pick you both up from your homes (there will be a charge,but not as much as a private taxi) in a mini bus ....it is not so much charity as a chance to meet other people for lunch and a chat and a reasonably priced meal you could both pay for separately ! She/you may not like the idea of a lunch club but it might solve the problem, If you suggest it to her anyway, see what her reaction is ,,,if it is negative tell her you simple do not have the money to pay petrol and a meal for you both and that your strength is not what it was ........if she has no sympathy or empathy and only wants to feel sorry for herself at these new arrangements, you simply have to say sorry ,,,give me a call if you would like to take up one of these ideas,,,,leave it at that and see what happens ! Sometimes you have to be cruel to be kind !