I too avoided the cycle path mainly because the road was more than adequate. It isn't far in terms of miles anyway to Marazon and on my route I was going to go somewhat north from there anyway to go through Dartmoor to Exeter.
All in all, in my experience, all roads are busyish and you just have to get on with it if you plan to ride from one end to the other. A road that is quiet to day can be as crowded as hell the next. It's Hobson's choice really which route you take.
