For many cyclists, riding 100km is a pretty big deal. In Zwift, it also awards a badge that is worth 1000 XP and a grey 100km kit. Here are a few of the most efficient ways to ride 100km.
1. Ride with a pace partner
There are tons of pace partners that are running throughout the day, and there is normally at least 1 that is on a flat route. One of the most efficient ways to get miles in fast is to join a pace partner at your level. A list of the pace partners and their routes can be found here: https://forums.zwift.com/c/futureworks/pace-partners/71

2. Join a group ride
On weekends, there are 100km group rides that are held every so often. Some notable ones are the KISS training rides, Dutch Diesel 100km, and the HERD M2C2. This helps you get the maximal draft effect and these rides are typically on flat routes.
3. Ride a flat route
While it may be boring, it is the most effiecient way to gain miles fast. Routes such as Tick tock or Tempus Fugit are great routes for gaining miles fast. I remember I rode my first 100km on Tempus Fugit, and I know many others who prefer Tick Tock for a change in scenery every so often.
I hope these tips are helpful for you to help accomplish a 100km ride!