I'm also 62+ also. I find it easier on my legs if I stay around 90 RPMs or more on the flats. On hills it varies depending on the steepness and length. But, I try to stay above 70 RPMs if I can. Mashing low RPMs wears me out very quickly!

I'm back into cycling after a 10 year sabbatical. I've realized that I lost a tremendous amount of core strength during that time. I'm SLOWLY racking up miles and speed and can feel the strength returning. At this point I HAVE to spin at high RPMs because there's no underlying strength in my legs. I'm constantly moving up and down the sprocket to maintain my leg speed.