I'm going to give Harpo a text in the morning after checking out the road conditions, hopefully it won't be too bad in the A.M.
That's an awkward one Rob, I'll happily go part way around Coryton on my morning commute (before 07:00) but outbound is a different story.
You could take Longwood Drive to Iron Bridge Rd, if the link works the highlighted route is cyclepath:
http://tinyurl.com/kk7gqhsWith that route you're trading one cycle path for another through the centre of Coryton, although there never seems to be glass on the section above.
Not that you would know until you get on to Coryton that time of day, but if there doesn't appear to be much traffic on the road I would have thought you would fine looping around to the Tongwynlais Hotel Junction, no way would I fancy joining the A470 and then getting off at the Morganstown/Radyr Exit.
Alternatively wimp out and go via Llandaff -> Radyr.