Something I've noticed after watching my nephew for a week while my sister was out of town is that different people in his life offer different things.
My mom cannot get down on the floor to play with him, but she can cuddle with him and sit him in her lap. He absolutely adores her. He wears her out!
But she managed to watch him for the 8 hours I was at work during the day. Boy was she happy when I got home!
Nathan comes in the door and my nephew's eyes light up. Because he knows he's about to play "rough house" style. He'll toss him in the air, throw him on the couch, drag him over the ground, and they have a GREAT time. My nephew expects that from him. He doesn't look for anyone else to do this with him.
Then there's me. I can get on the floor with him, I can play and tickle, but nowhere near as much as Nathan. And I get worn out pretty quickly. But who was it that he turned to when he bumped his little head? Who did he snuggle with while I read a book? Who gave him his bottle and listened to his snuffling because his nose was stuffed up? It was me.
I know that I'm not as limited as you are, Ross. And I know that you're worried about giving your grandson the energy you think he deserves. But something I pointed out to Nathan when he noted how exhausted he was the first day of watching Wyland is that they don't come out walking. You'll have many months of snuggles and sleeping before he becomes mobile. And at that point, he'll be so in love with you (and you with him) that energy will not matter. He'll know what to do with you and you with him. Everyone in his life will serve a different purpose. And you will be there for him in every way that counts. Do not ever discount that importance. You will love him and he will flourish in that love.
Congratulations, Grampa! You're going to be great. And remember the best part about being a grandparent (or aunt or uncle) is that you get to send them home at the end of the day!