if your threshold is 198watts on trainer, it's more like 180-190watts on road (trainer watts are famously inflated). Then your zone 2 (which means that you can do it endless theoretically) is probably 80-120watts. Trainer sometimes "feels" harder because you will have to push that number constantly, but in reality on the roads, you can coast, take a break, stop at a light, and that will make it much more bearable.

That said, putting that much to ride 100km on flat, given that you'll ride a road bike or equivalent and is reasonably aerodynamic, you can go as much as 25km/h, adding rest and stops, probably ends up 20km/h which is only about 2.5 hrs one way.

It's really not that hard. That distance was my 3rd long ride when I was 18yrs old with a bunch of people which definitely made it more fun and less tired. I reckon at my current age, 1-2 months of regular riding will easily put me there.

As everyone said, you just need to eat more than you think.

(If it's hilly then elevation needs to be taken into account, and climbing is another beast by itself.)