The walk itself is just over 9 Km. and involves a height gain of over 900 m. (2,700 feet) which equivalent to climbing 270 flights of stairs. So you will need to be reasonably fit. It will take approximately 5 hours from start to finish.
At this time of year we can expect it to be cold, wet and windy. Fortunately the start point for this walk is sheltered to some extent by Cadair Idris itself because generally the weather rolls in across the Irish sea. But once upon the ridge line we will be exposed to the weather and it could be blustery !
The terrain itself lower on the mountain is a mixture of woodland and moorland. But as we get higher up this will gradually change to become more rocky and the top of Cadair is largely scree.