I can't be certain as I started organising before the current rules (and by the backdoor, inheriting the Cambrian Series from Peter Coulson) but I think it is a little easier than organising a calendar ride as all you have to do is put a route up, there's not so much worry about controls and risk assessments and things like that (again I inherited some good risk guidance on the Cambrian series from Mr Coulson - basically you are on your own, the roads are remote, will be covered in s**t and take it easy so you don't fall off) - if you don't have the Cambrian factsheet please PM me and I will send.
The hardest part, IMHO, is finding control points that take you along the way you want riders to go - time and time again I see a really great route but actually with little extra distance its possible to reduce 800m of ascent to about 250m etc etc. Now this may be more of a problem for the Cambrian Series which are designed to be hilly as well as being a nice route.
I do a combination of research via Google Streetview, broader websearches and my experience of riding on the ground to try to find suitable control points. At least with 100km rides you can use local shops/ cafes/ post offices as controls which may have limited opening hours (eg 10 - 6) but for the longer events you really need ATMs or 24 hour petrol stations of which there aren't many in Wales
And places close. Llansannan is a well placed control in North Wales for a hilly and scenic route (Cambrian 4F and 8A)but the last rider through reported the pub and the post office shut (no doubt the light switch will be turned off soon too).
The Perm Sec has been very helpful in agreeing rides and will point out flaws etc. We had about 3 rounds of correspondence on the Cambrian 10A and he was very sensible.