I would say first of all well done to you already, surviving cancer is a huge triumph.
My advice would be to get yourself out on the fells as much as possible, gym work is fine for cardio, but there’s nothing to replace hill training.
Unlike the gym where it’s smooth and you’ll train one set of muscles constantly, out on the fells it’s rough under foot, undulating, you use certain muscles to climb the fells and others kick in when descending, only to start those climbing muscles again and this is where hill fitness comes in.
Constantly changing muscle groups is energy sapping, coping with a constant undulating ground is much harder work than road walking or cycling and 8 years break is a while.
I’m sure it’s possible for you to get fit enough to complete the 20 miles, but don’t underestimate it, it’s a big challenge for a none Fell walker.
See how you go on the hills in the next few weeks, you’ll get an idea.
Maybe go for the shorter option this year, enjoy it, gain some experience, know what you need to do and have a pop at the 20 mile next year.
Whatever you decide, all the very best and good luck