To your question....it's absolutely unrealistic in your case. If 61 mph is twice as fast as you've ever gone on a bike...you'd know without a doubt from using your god-given senses if you were really going anywhere near that speed.
Yes, I agree with that...and I'm pretty sure that an 8% grade isn't enough to yank someone along at that speed!
A few weeks ago I hit 60.4mph coming down Valley Mission Rd. on the south end of Lake Monroe (IN), and while the entire segment averaged 8-something% over the .7mi segment length, the upper portion was considerably steeper, bettering 23% grade in spots, and I was rolling pretty fast into the start of the segment.