Apologies for bumping an old thread.
Have a better idea now. Did 291 miles today from home to Devon. Roof down, one stop after 1 1/2 hours and 114 miles and then another 3 hours later and 250 miles into journey. Despite various joint issues have managed to avoid walking like a chimp although a little tired and stiff. Ears ringing a bit due to wind and sports exhaust but certainly not unbearable. Lady HB reckoned she’d had enough after 200 miles.
So the answer to my own question is 5 1/2 hours and 291 miles (though I’d be willing to go for longer personally....😇).
....in case anyone was interested....
HB
