If you are starting on Pen cerrig-calch and finishing on Pen twyn mawr I did the same walk last weekend, I thought it was pretty easy as there is hardly any hard ascents to speak of and the ground is good underfoot apart from Waun Fach as you mentioned, but that sort of bog is pretty standard stuff in a lot of places.
And I nearly knocked myself out on the door frame in the pub in Llanbedr, they had a nice IPA on in there which name escapes me, it was almost transparent, but tasty
If the route uploady thing on here ever works I will upload the GPX file of the route.