As a health visitor, I saw this a lot, and would advise:
Your age isn't important, but your health & vitality are. make it very clear that you will do your best, and let them know if you feel you cannot cope.
Keep everything simple, don't worry about using ready prepared meals etc.
Look around your area - there will be toddler / stay and play type groups that welcome grandparents (indeed, you will often find they outnumber the parents!). Use these to give a structure to your day.
Do you like swimming, and is a pool nearby? We find taking LOs swimming is a lovely experience, takes up a good bit of time, excellent exercise for us all, and usually tires them out!
If you decide to help with nursery fees instead (I obviously don't know your situation) then consider - depending on the geography / transport - caring for your DGGC for 2 half-days rather than 1 whole day. You may find this less tiring.
Nurseries are expensive - in this country we have a perfect storm of high standards and minimal state help. I applaud the high standards, and would like to see nursery staff paid more, but under the current arrangement we end up with this situation.