Having spent far too many hours in the last 7 years volunteering around canals as a volunteer lock keeper / towpath ranger I think you've hit the nail on the head when you talk about pacing yourself.
My observation is that the journey suggested by the hire boat base may have been realistic for an experienced (and fit) crew, but for a newbie (or an elderly couple for example) it was plainly wrong. It's a bit like a bike hire company hiring a bike and telling someone to go off and do LeJog.
Unfortunately the Hire Boat Companies seem to sell dreams - they will paint a picture of lazy days floating along on a haze of wine fuelled calm - when in reality it can, as you've found out, be bloody hard work. It doesn't have to be a hard slog, if you've got a good team, and a realistic idea of what you want to do (decent weather helps as well). As you rightly say, the parallels to cycletouring are there to be found.
Hire boat bases seem to vary enormously in quality - some give almost no safety instruction (canals and particularly locks are seriously dangerous places), some give little advice around how to control the boat, how and where to moor up, and the advice on boat safety (Carbon Monoxide in particular) can be woeful - there are exceptions, and I sometimes wonder if it depends on who does the handover, rather than the company or the base.
Many people live on their canal boats (narrow or otherwise) either all year round, or during the summer - and are jolly happy doing so. It's not for the faint hearted, particulalry as winter approaches. I heard a story last summer from a couple who'd sold their house a decade ago to move onto their boat full time - they had thoroughly enjoyed their first 7 years or so, then the wife became ill, but because the boat's value was depreciating, and house prices were going up, they couldn't afford to buy a house, and because they were 'of no fixed abode' they were having major difficulties getting a local authority to home them.
The Llangollen Canal was the scene of our last narrow boat holiday - and a really pretty waterway it is too. Maybe best seen at this time of the year when it's not too busy, and the autumn colours are coming to an end. The area around Ellesmere, and its meres, is also very pretty from the canals.
