
When I lived in Cornwall my dogs would run through the sand dunes where there were many adders. I asked my vet what I should do if one of them was bitten. She said that with larger dogs it was usually no worse than a wasp sting, so a vet visit was necessary but was not an emergency. However with a smaller dog the venom could have a worse reaction as there was not so much body weight, so this could mean a rush for treatment. To not panic as this would upset the dog, and cause the venom to travel round the body quicker, to carry the dog if possible and to keep it quiet on the way to the vets.
Luckily the sand dune adders would try to avoid the dogs if they could and I never had a dog bitten in the years I was there.
It would be a good idea to find out the local vets and their hours of opening and also their out of hours number, but before going have a word with your own vet to see if there is any emergency medication you can take with you, that could keep your dog going if you had a bit of a drive to the local vets.