Wow! Some achievement.
I grew up near Cadair Idris and rode it a couple of times in my youth. The BBC's decription is slightly misleading as the path is legally a bridleway all the way from Llanfihangel to the summit. It just progresses from modern farm track to old school green bridleway to grassy single-track to steep rocky path. The latter having sections which are almost easier to carry over than ride on the way down, never mind the way up!
The descent is unbelievably good fun. Cadair Idris is second only to Snowdon in Wales in terms of the number of people climbing it each year. However the "Pony Path" from Llanfihangel is probably the dullest of the many ways to walk up meaning that mountain-bikers don't encounter as many walkers on the descent as might be expected. Still need to keep your wits about you though.