Use this expression for the “Source Text” parameter of the text layer.

pointA = thisComp.layer("Null A");
pointB = thisComp.layer("Null B");
wordL = " "; // The word before the number
wordR = " "; // The word after the number
dig = 1; // Number of digits after the decimal point
""+ (wordL)+length(pointA.transform.position,pointB.transform.position).toFixed(dig)+""+ (wordR);