If you're doing the route on your own separately, you just need to select the Watopia Flat route (10.3km/6.4m) and do THREE laps (not 4), which - including the lead out from where you're spawned - should be just under 32km/19.9miles. There are a number of segments already set up to cover the whole 3 laps combined - I suggest we use "Flat Route Forward 3 laps (Checked)" -
https://www.strava.com/segments/12211763?filter=club&gender=all&club_id=126532
Probably best if you warm up elsewhere, and then start your ride in Watopia Flat, and ride flat out from the start (don't wait until the banner just in case the segment starts slightly before it), and as long as you've ridden for 32km/19.9 miles you should have definitely completed the three lap segment that we'll take the time from. (I recce'd the course yesterday and I'd completed the three lap segment after about 31.5km from where I was spawned).
Remember to do it on a TT bike too.