Yeah, I wouldn’t say you need a training plan as such. I see people riding it who look like they haven’t been on a bike since they were 12. And there they are on their Tesco full suss….seriously.
There are some pretty serious hills however. Many people do walk them! Depending on what you’re used to, they may come as a shock. Or they may not. You’re traversing some of the highest points the country has to offer without getting into mountain territory. If you’re on a road bike you might want to think about gearing.
Just ramp up the miles best you can. If you’re pushed for time, I find even a 20 minute fast-paced ride out on an evening can do wonders for fitness. Get a longer ride in when the weekend rolls around and you’ll be good to go.