Hi and welcome! Do you mean long distance as in 'long running'? If so, then this layout is quite long, we've timed a train at about 25 minutes to do one complete circuit. http://www.irishrailwaymodeller.com/showthread.php/795-A4-Mallard-s-new-layout-Progress-so-far
As for longest straight? I have no idea. More a 'gentle curves' man myself!