This old man wants to stop feeling tired

I always told my kids that after I retired I would be so well rested that they would always have to check to see if the old man was still blinking!  Sadly my experiences since I retired in June of last year have actually been very different from that.  I have found enough hobbies and interests to fill my time, but my main issue is just that I am tired, and I would give a lot to stop feeling tired.  It means that I am not really able to enjoy time with the grand-kids, or even to have the fun I though retirement would bring.  I hope that there is a good solution out there.