Yeah, the various internet route trackers round here keep wanting to take me off the main road and onto the canal tow path, the latter consisting mainly of mud with rocks in, with intervening locks which consist of wet cobbles. Completely uncycleable on my slick road tyres (though probably a laugh on a properly set up MTB). None of the route planners I've found let you input what sort of terrain you're set up for.
Wish there were more paths like that everywhere, as it's just a wide smoothly pathed track, and no traffic. Win/win.