Mike's reminded me of when, aged 19 or 20, I tried to ride from Stroud to Bath on the A46. I got as far as Dunkirk/Petty France, which is actually probably over half way, before turning round. In fact, a bike with water bottle cage bosses would have been helpful.
Ed: The A46 between, I'd say, Nailsworth and the M4 junction, is actually not terrifying at all. Nailsworth to Stroud is a bit narrow and twisty, M4 to Bath is much busier. But the bit I actually rode along wasn't really that bad, it was just the close, fast passes which I wasn't accustomed to on top of lack of water.