I think you're safe tomorrow

Been playing with mapmyride some more and found that it does display large grades of 20% or more, which is good. However, the noted grade is not accurate. I did a small route over a local bridge which has almost no change in altitude as you cross it. However, when I do a route over the bridge, it drops me down 100, then back up again.

Good luck on your first 200K tomorrow!