Training involved to ride 175 miles (281 km) in a day
I'm in the do it this year camp.

In comments you say you can currently ride probably a painful 60 (miles).

In general, a conservative plan is to extend your ride distances by 10% per week. But it's important to realize that marathon and iron man athletes do not train by running a marathon each week. The aim of their training is to build deep core fitness and endurance, so that on the day they have the resources to push beyond their normal levels.

You have 15 weeks to RAW day. If you start now, with a 30 mile (50 km) ride, and do a 50 mile (80 km) ride at the weekend, you'll be on your way. See this answer to a somewhat different question for the kind of plan to follow. With such a plan, in 7 weeks you could be riding 100 miles (160 km) in one ride on the weekend. Yay! All being well, health and injury wise, you could keep extending the distances, but that would be unwise. I suggest that when you can ride the century, that your long ride should be every other week, and as you extend the long ride, reduce the distance of the rides in the next week, to keep the training load constant. Limit the maximum ride to 8 hours in the saddle. At your pace that would be about 120 miles (close to 200 km). That would be about 16-20 hours training per week in the final weeks before you taper over the last two weeks. Oh, and remember to factor in recovery rides for the day after the long rides, especially when they are longer than about 90 miles (150 km). At that point you would be training 7 to 8 times a fortnight (light weeks and heavy weeks).

The RAW website is still being built, so everything is not clear yet. It seems likely that this will be a supported ride, but as I write this it doesn't say that. If it is supported, then the ride will be much easier. You can stop for refreshment at each station for just a few minutes (not too long, you don't want to cool down). One thing to work out on your long training rides is what food and drink works for you. Since you are not aiming at extreme performance (15 mph, ~25 kph), you don't need to use fancy / exotic food and drink. You must, however, eat and drink small amounts frequently, and right from the start.

Another thing to consider is your bike and other gear. Does it need maintenance? Can it carry two drink bottles? Are you practiced with tire repairs? Do you have a variety of cycling garments, especially a lightweight rain top? The latter is valuable as a windproof top, eg if the morning start is cold.