Hi Andy
I am much like you and love the distance stuff. Currently I am doing about 16km, 8km into the wind and 8km back downwind. I am doing this in about 4 hours with rest stops and time to explore.
When I first started I remember doing 1-2k and being exhausted but pretty proud of myself. Then I started to get really into it going out 2-3 times a week whenever weather would allow. I have no formal training regime, I just look for time on the water and make sure I am having fun exploring new places. There are a couple of things that are helping me though.
The first main thing that got me going further is technique. Look at you tube before you go out and practice what you see. I really like this one
The other thing I do is look at more accomplished paddlers on the water and try to copy them, just try not to be too creepy about it!
I was amazed at the difference proper technique can make.
The other thing that was a revelation was my Camelbak Baja. I was maxed out at 5-6km because of hunger and thirst. Now I can take lots of water, food, sunscreen, phone, etc no worries.
Like I said earlier, the most important thing is to keep having fun and don't turn it into a chore.
I am currently looking at a distance board so I will make sure to post any thoughts I have from demoing whats out there.