To get to Ultimate Time Trial
Single Player > Free Play >Map Selection (Level) > Event: Ultimate Time Trial
I struggled to find it. So decided it might help someone else.
The King's Garden
Achieve 1 (single) lap under 1:30:00. Added extra emphasis here because I thought it was total time and kept retrying Parking Lot in a 3 lap trial. Same with Backyard Track. Kept retrying the race
Now for the reason you're here.
I used 150cc bike (Little Dipper) an achieved a time 1:20:00. With falls 1:25:00 at the most. There is a secret hole other people have commented on. There is one. Guide over. Jk. After taking 2 90's left turn you enter a wide open platform. LANDMARK: RED TOWER. Take the long jump over to get to the wooden under construction platform. Upon leaving this wooden corner, drive straight down to the open hole ahead of you. You'll drop down and see, a backhoe and a dirt mound. If you are driving down a dirt mound and see a backhoe in which you can drive underneath. You've driven too far. Furthermore, if you are still having trouble finding it. Go to standard race with 10 laps to explore. Tried following these other guides, still took me 20 minutes to find exactly what they were trying to describe. All in all, shortcut is around 0:50:00-0:55:00 [50-55 seconds into the level] Another hint, Press(A) to power slide 180 turns.
Achievement Unlocked upon crossing the finish line. Very wordy guide, other guides were too vague for me to follow. Hope this helps.