My first downhill experiences were on the back of a tandem with a husband driver who is used to going fast. I know he went slower than he normally would, but I'd still be yelling, "Scary! Scary! Maybe we should slow down!" Eventually, I stopped saying that and the downhill speeds just got faster. Our fastest yet was 50 mph on a short stretch. Talk about exhilerating! A tandem is heavier and more secure feeling, though. The fastest I've gone on my single is somewhere in the mid to high 30's. Maybe 40 at the most. No need to push it past what you're comfortable with. Good for you for working toward conquering your fears.