The start control of my Audax DIYs is at a local Tesco ATM. It's 2m lower than the highest point in Solihull. Luckily, every direction from there is downhill. Not steep but lets me ride fast enough to post a good average in the first 5 miles of the DIY. In whichever direction the DIY goes, I can clock up 14 mph average at the first control 18 miles away. I work hard to maintain this average, or at least 13.5 mph. The final stretch is ALWAYS up a slow gradient back to Tesco, but by this time and distance, a good average is accumulated and 12.5 mph is easily achived even if I dawdle at 10 mph the last 5 miles.