Welcome to the forum, Ken.
As long as you have no underlying health issues, there is no reason why you could not achieve your 23.5 mile walk.
I don't have any specific advice other than to get out and walk! Gradually build up your distances as you get fitter.
Good luck
Hi Ken welcome to the forum from Shetland.
I agree with Richards post, quoted above.
You have given yourself a great target and a sensible timeline.
The key is to enjoy it at your own pace.
Where to start depends very much on where you are at the moment in terms of "walking Fitness"
I have been involved in coaching athletics since the seventies, and an active walker/hiker since 2015. If you want to contact me personally regarding your current level of fitness I would be happy to advise a training guide/program for you. In many ways it is like any training program, in that we would look at what you want to achieve and when you want to achieve it by. Then work out a training program/guide backwards from there, to wherever your start point in fitness is at currently.
It wouldn't need to be anywhere as heavy as a marathon runners programme, but the journey would be similar. First few months achieving a base fitness and just getting in the habit. last month or two keeping up a good mileage @90 mile a month, slowly extending your longest weekly walk.
Great to have you on board.
PS don't be concerned about speed people walk at. they all measure it differently. but that is another discussion.