You need to do at least more miles than your average figure to have something to talk above. i.e. do 18+ miles at least at with an avg of 18. or 20+ averaging 20. It is funny, unless of course in a competitive TT, to hear guys posting an avg of 19 miles over 6 miles. A performance target of 25mph would be ideal to work towards i.e. a 4hr century, but very tough to do solo, usually accomplished in group. Many racing recumbents though can avg 25mph solo for the century.