Your thread is a little confusing as you also searched for "Mountain", but Im assuming you are asking about the Ironman. I have completed the Ironman twice so I will speak from my experiences.
The canoe portion is 8km and is completed like all of the Ironman on an individual basis. I dont remember the exact size of the canoes, but they are your standard two seat canoes that you can rent from any camping or outdoor store. Most people sit in the front of the canoe facing the rear; this is because the canoe wont sit so much out of the water. And yes most people will place the ruck in the opposite end of the canoe to balance the weight.
As for workup tips. These are my own tips and your mileage may very. I rucked at most twice a week. Spent the rest of the time doing long distance running (up to 26km) and some cross fit.
Its very important to work your feet up to the distance required for the Ironman, but just rucking wont get you all the training you need. Squats, lunges, and anything that works your back and core will help out a lot as well. You will be carrying a lot of weight on those shoulders, then youll get to the portage. You dont want to be worrying about how sore your feet AND how sore your shoulders and back are.
And for me the two biggest things that cut my time on my second race, was getting a lot of time paddling to bring my time to under an hour for the 8km canoe portion, and practicing portaging the canoe AFTER a long ruck march. Me personally, I find portaging pretty damn easy with a ruck on. But its not the same thing after 32 km's.
Best of luck training.