Oldandpastit, I think you are having a bad time here, but I will give some support. How well you are and how you are able to cope, is not just governed by age, unfortunately some people are struggling to manage and lacking stamina at 60, others are climbing Everest at 70. I do not think age is a factor.
You say you have had a cold, if by that you meant the really bad cold and cough, the 'dreaded lurgy' that is doing the round, you have all my sympathy. We have had several threads complaining how it leaves you totally without stamina for weeks after the cold and cough have disappeared, and anyway if some one is ill, the last thing they want is someone visiting with an illness that they do not want to catch.
Your daughter does seem to have been in the wars in the last year and she must really have been struggling. Even if you cannot visit her, have you been keeping in contact with her on a daily basis to see how she is. A daily phone call to check she is OK, to let her talk - her cancer threat must have been very distressing, especially if her children are young. Are the children old enough to travel on public transport. Could they stay with you for an occasional weekend to give your daughter a break. Could you manage a weekend with her every month?
I think you need to sit and think around the problem, your daughter is going through a really difficult time. Your energy may be limited but keeping in daily contact, if you are not already will help, and perhaps talking to her about what you could or can do rather than what you cannot do would help as well.
I am sorry but your posts have come across as very hands off rather than hands on. Perhaps being more positive in your attitude to her and the problem will suggest solutions.