get a crowd would be good fun! I think quite a few would if it was organised properly.
I'd anticipate you'd need about 5 full days walking to do...route would be good...I've done most of it in bits, the problem is the Severn.
Black Mountains are OK to cross as its more or less a plateau once up, Brecons would be more challenging to find a reasonably direct route, as there are some big old lumps around.
Being organised is not something I ever get accused of.