You need to do some route planning on www.viamichelin.fr/
A drive from the tunnel to the Med is 750 miles and should take about 11 driving hours; going to the Atlantic will add another hour. Fuel is only half the cost as motorway tolls are almost the same again, about 200 euros in total. Some free roads can be quicker and have less traffic, which is why you need viamichelin.
You also need to get a telepeage token so that you do not have to pay as you go, and therefore have to queue to collect and pay for motorway tickets. Most big exits have a rolling 30 km lane to help you keep moving and are a boon if you are driving a right hand drive car. Look for www.aprr.fr/
A word of warning about French motorways, they become free ring roads around the major towns and thus much busier and often snarled up. Today the queues totalled 500 miles throughout France as people were coming and going from their holidays, but 3 hour queues, around Bordeaux for example, are not unusual during the working week.