It’s not continuous though…
"At 292 miles (470 km) long it is the longest ‘A’ road entirely within England, and in the United Kingdom as a whole second only to the A1." Says Wikipedia.
So it’s not even the longest, but it becomes the M5 for part of the way – so you couldn’t actually ride all of those 292 miles anyway. The same goes for the A1.
You could ride the full 274 miles of the A74.