Let her live her life, for god's sake. 56 is way too young for you to start mothering her. I'm sure she knows the roads as well as you do. And cut out the sexist crap. We'd achieve nothing in our lives if we believed the "it's different for women" crap that we are continuously being fed.
And there are promising studies that exercise reduces the risk of reoccurrence of cancer, so it may even save her life directly.
When my mother was about your mother's age, she expressed a wish to cycle again, which she hadn't done since she was a teenager. I went to her house the next weekend, but my father who had a fear of cycling, had talked her out of it. 20 years later, she has a massive, debilitating aneurysm with extensive brain damage. My brother asked what could reduce the risk of it, and the neurosurgeon said "cardiovascular fitness". If my mother had taken up cycling again in her mid fifties, maybe she'd still be around today. She certainly had the genes for longevity. Several of her siblings made it into their 90s and her 96 year old sister is still living independently (though not a cyclist

).