For a seven day trip to the Basque region, we were wondering if it can be combined with the Asturias region. Or should we just stick to Basque? At our age, there's a good chance we're never returning (too many other places in this world we want to visit).
Personally, I wouldn't split the time, but that decision is driven by my interests and my ability to take long trips. What sort of sightseeing are you interested in?
It doesn't help that public transportation in that part of Spain is not fast. If you're contemplating traveling by rental car, you can check estimated driving times on ViaMichelin.com.
Consider also that you may find it cost-prohibitive or logistically inconvenient to fly in or out of an airport other than Madrid or Barcelona. If you have to use one of those gateways, a good chunk of your seven days will be used just to get to your first stop and to return to your departure airport from your last stop.
It would be easy to spend all your time in the Basque area. If you plan on visiting small villages a rental car may be a good option. Between the Basque area and Asturias is Cantabria which shouldn’t be overlooked. With only 7 days there really isn’t sufficient time to adequately cover those three regions.