Depends on whether you get lost / breakdown / stop for flower pressing or hoon at prodigious speeds between Santander and 'the other post'.
Put your departure and destination points into something really simple like Google maps and ask it for a route. This will be the software's best go at estimating a non-stop time. If you like its answer, great. If not, think of something better or an alternative.... or give up the foolish pipe dream.
Do the same thing again in something like ViaMichelin and compare the results. Repeat in other routing software until you get an answer you like or lose the will to live.