Hi jonathanC,
Spewin you are in melb! LOL we are enjoying great warm sup conditions up here at the moment.
The fitness sessions will be run over a 1 hour time, so as far as we can push 30mins and then turn around for the last 30min, i'd like to think about 10km round trip with no wind assistance.
As a personal trainer i can't recommend cross training enough for long distance paddling is also very important, such as 2 or 3 weight training session per wk.
There is no easy way out of building a good strong fitness core, so it's hard work only but fun as well, especially when you get a group together as people like to compete.
I have got average ladies joining our fitness sessions, stealing their hubbies boards and dumping their kids with him LOL.
Doing push ups or situps on your board every 5 or 10 minutes is also a great exercise. Also jumping off into water and getting back up on your board, do that 10 times in a row and you will feel it. Also think sprints 1min hard then 1 min easy do that for about 20 mins.
Wazza might jump on this topic and give some correct paddle tech aswell.
Hope this helps you out abit. Have fun doing it aswell.
XX Angie