I found the easiest way to do this was by building your own level. Firstly, you'll want to have unlocked Forest world in Career. Once that's done, you should be able to go to Start - Ride - Freeride - Custom Track. Once in that menu, select Forest as the world, none for visual style, none for gameplay modifier, infinite for health.
Make Steepness maximum, curves nil, stunts nil. To help, you can build a crew. I used Heavy Bail Threshold, Fewer Obstacles, and Speed Wobbles.
Then select Build Track With These Settings. Hold RT and try and take the steepest way down. My first track didn't have a steep enough area to get the speed required (and none of my Highlands tracks were steep enough), but my second Forest track was plenty steep and I knocked out around 107km/h.