I grew up in Cornwall and yes its ruddy hilly. My partner decided to start get fit and the begining of lockdown and is a mid 50's lady . She does not like riding on the road and her aim is to race Cyclocross with me come sept/october. Luckily I had 2 turbo trainers before lockdown and also power meters etc. We have done 80% of her fitness work on turbo trainers. The other thing she is not mentally competing with me on a turbo, just herself.
She has made vast improvements in aerobic capacity and power. Now when we go out she can manage every hill in our area bar one. Hills she could not ride before she now goes up non stop.
So I would recommend Turbo training, the same as
@biggs682 it is generally considered that time 1hr on a turbo is worth something like 2 hrs on the road ( though no doubt someone will argue wth this .
Also my brother who is 62 and lives in Gweek and has not rode a bike since his 20's was furloughed and bought 2 old clonkers and made one out of them and has started cycling again.
Beware of other non cycling people tellling you that your mad trying, my partner gets this a fair amount from her family and work colleagues.
Good luck and keep at it!