I've done 59.5mph (60 near as dammit) - the speed was ok but I started to get shimmy which was my saddle-clenching moment. I didn't actually know how fast I was going otherwise I'd have been more concerned - not least because I was a very long way from a hospital (I was in the Cairngorms)
I'm guessing this was on a downhill stretch of the Old Military Rd?
I've just recently returned to cycling and couldn't be further from peak fitness, but I hit 43mph there a couple of weeks ago. I can't describe how exhilarating it was.