Here is a completely different suggestion
Rather than ever increasing challenges, which for me sort of ground to a halt after the London Marathon. I found the training too time consuming and as I'm now over 60, I'm not going to get faster.
I now use a game on
Fetch everyone. It was primarily a running site but now has a lot of bicyclists. It's basically a forum and is very good for logging/analysing training
The game sounds silly and is hard to explain but it gets me out. (there are others but haven't tried them)
Here is a map of the area near me.
It is all about gathering points. Bugs are bad and 'cost' points, those that are nearest to your home cost more. They have to be moved to outside the mile area, (and carried a mile from where they started to be 'squished' If you go into other peoples areas you can pick up their coins. If you go out of your area then you plant flowers, if you go back they are watered and eventually get harvested or picked (help this gets complicated) You can either download your training from a GPS watch or similar or plot it by hand.
Basically it makes you move a lot within your area but flowers also gain points and the bicyclists tend to be the leaders as they go further afield . It is deceptive though because you can end up going a long way, even if you don't stray far outside your circle ( you can log walking as cross training)
Because I'm not in the UK, there is no-one else near me but most places in the UK have fetchies so you can pick up other peoples things and plant flowers that they may pick. (and get points) I suspect it's a bit easier in towns because a lot of my 'coins' are inaccessible, though you are allowed to get things moved if they are dangerous or inaccessible/ I do it for bugs but not for coins. Things change a bit every night and everything gets reset at the start of each month.
At the moment for July I am 254th but the leaders are all cyclists and all have many times the number of points that I have.
Here's a map of somewhere in your county.
You can see that there are at least 4 other people in the area as there are 4 gore(illa) markers, you only get one which moves each day. You don't have to connect with these others in anyway thouth.