Been there, done that, but behind a motorcycle. I did 60 mph on regular non-blade roller skates. I used to do up to about 45 or so on the downhills, without a tow, so it wasn't far outside of something I was somewhat used to. The extra 15 mph was a big stretch though, as well as the difficulty in adjusting to having all the towing force from being pulled through the wind by your arms, vs just gravity doing it. The scary part was that it would be real dangerous to just let go of the bike at that speed, because of the wind turbulence coming off the back. As I got more and more tired, it became pretty worrisome about what might happen if I either tripped or let go.
Just another bit of insight into the weirdness of Zippy.