Good morning Grant,
Welcome to where two hands are better than one, and now when people ask how big is your rod, you can say, I need two hands to hold it

The constant suggestion of 'go slow' is the key to using Doubles this is simply because of the rod's length, which now results in a greater tip speed EVEN when your hand and body movements are slow.

DO NOT attempt to drive the rod like a little single handed rod but in its place allow the beautiful Double to work for you, which again includes slowing down your actions and the Double will come forward with all it's powerful and graceful energy.

DO NOT buy a long belly nor mid belly Spey line, in its place purchase a short belly line at about 50 to 55feet as this will be far easier to learn and control.

If your having problems with the cast, after going slow, I would suggest to most people that they should first consider their leader length, because with Spey type casts the leader forms the anchor, the anchor is the tool that in turn loads the rod for the cast.

Fail to form the anchor correctly and your cast will fail, simple as that.

AND forget all about distance casts till you are casting smoothly and with total confidence
AND then the distance cast will come with ease, without thinking about it.

Again welcome to real fly rods.

Kind regards,
UB
ps, I will bet you within 12 months you buy another beautiful Double Handed rod