I'll be 56 when my eldest turns 18 and 57 when my youngest turns 18. Oh well. Who knows if I'll live that long anyway? There are no guarantees, and my DP is 15 years older than me, so he'll be in his 70's, but let's be straight here - we could all be hit by a bus tomorrow so what's the point in worrying about things that are almost a lifetime away.
Am I worried - no, not at all. I'm a much better parent than I would have been in my 20's or early 30's and I'm with a partner that I'll spend the rest of my/his life with. Had I had DC to anyone else, they would, quite honestly, have had an asshole for a Dad, as everyone else I dated really was not someone suitable for parenting my children. It took until my late 30's to meet "the one".
My Mum is 69 and in great health, much fitter than me, and could probably keep up with my DC easier than I can. 60 is the new 40 op. You'll do great.
And to whoever felt the need to mention that you'll be 60 when your DC is 18, well sod 'em!! You don't need to ask for permission or approval. You do you, and be happy. Congratulations on your DC op.