Does a car run more efficiently if it is used more often?

I bought a second hand car recently. It had been a second car for the previous owner and they didn’t use it very often. It has very low milage for its age. I filled up with diesel and reset the miles per gallon counter. I then drove to work and back on my normal 50 mile round trip until the diesel got low (about 2 weeks). At this point my mpg was 48. Filled up, reset counter. This time I got 50mpg. Subsequently it seems to have settled at about 51 mpg. Has driving my car regularly and on the motorway been good for it? Made it more efficient? How?