I am new (and old, 49) too.
My adult life has been spent on bikes but I am only getting serious now.
I understand the desire so many have to keep healthy and enjoy the kids. I have been driven to fitness for the same reason.
I had to give up a 10k running habit when I developed severe plantar fasciitis. I have a problem with scar tissue development and may never fully recover. Biking has gotten me out in the fresh air again.
I also get to spend more time with my husband. He has been an avid cyclist since his 20s and he is enjoying riding road bikes with me again.
Enjoy your rides with your grandchild. We have biked many miles with our boys. First with them in bike seats and then with them on their own bikes. We have many happy biking memories and wish the same for you.