Just to give some feedback on this topic, I ended up cycling from West Wales (near Newgale/ Haverfordwest) via Aberystwyth and Builth Wells to Cardiff. All in about 220 miles in 3 days. I used a combination of routes 81, 84, 47 and 8
With the combination of hills, heavy rain, strong winds and off road ( Wales is good for mud) it was bloody tough. I got the train back on Wednesday night from Cardiff to Haverfordwest.
I was impressed with the train service.
I'll post up a full ride report if anybody's interested.