Learn to ride again, why 40mph?
I have been to the south of France,Borders of Spain, To Holland, France, several times on a pre war 500.
Avoid fast A roads, motorways, Don't even think about keeping up with modern traffic.
In fact for years its been the only bike I'll ride for such journeys on.
A 3 day trip sounds ace
go for it but pick good weather, if we ever have any again

Its less than 40 miles for gawds sake.