Great to hear other grans get exhausted too and not everyone is a supergran.
Anniegran - you were blessed having a family that pulled together. Our parents never helped which made me want to be a hands on grandparent. I carried on working but only part-time and in jobs where hours were flexible so I could usually be around at end of school - even (especially) when they were teenagers. I still had time for 20 odd years of a successful career in my own business - helped by what I learnt as a mum.
Waller 5, it sounds like you have answered your own question. If it is phycially too much, it is too much - and unsafe too.
We help with grandchildren in lots of ways but I quickly realised that physically I had to set boundaries when it came to daycare (&, Waller5, I'm only 65 ) & clearly communicate these to the parents. I only do 1 regular day a week, mainly after the age of c 11 months and at first only one at a time, rarely before 9 or after 5. Daughter couldn't afford childcare so gave up the job, trained as a childminder (does 1 day a week) and manages husband's business from home. Now the grandkids are 2 and 4 it's easier especially now grandpa has realised how much fun we were having and comes on our jaunts too.