If you are going to avoid motorways then you will not average much more than 30mph over any sort of distance. So use that to base your daily distance on, for the first time driving a M3W you will find 180-200 miles a day plenty, the longest drives I have done are 440 miles on motorway and 300 miles non motorway, you will find it much more tiring than a modern car. The UK school holidays start this weekend so the roads in the popular holiday areas will be busier than usual for the next 6 weeks.
