For me the term “tough” is not the best way of describing a walk. It has a burden of implied machismo which is inappropriate for what should be a pleasurable activity. I prefer terms such as easy, moderate or challenging.
The factors that would make me describe a walk as challenging would be the distance covered with respect to: the going underfoot, gradients (both up and down), height gain, weather conditions, psychological factors such as exposure and the ease of route finding. The speed at which it is walked is a matter of personal choice. Any walk can be made unpleasant if you go at it hard enough, everyone has their preferred pace.
When I publish a route, the only factor I ignore in assigning a degree of difficulty is the weather on the day, though even that will get a mention if the walk should only be tackled in certain circumstances or affect the going. I never suggest how long it might take - that is up to the individual. Walking is about having fun, not beating the clock.