TR workouts are structured in such a way that the intensity is much higher so that you dont have to ride as long. You’ll get the same, if not better results from working at a high intensity for two hours than spinning away for 4.
For example you’ll see a lot of workouts in the .80-.90 IF range over any where from 1-2 hours. If you compare this to the pacing required for your century which will likely be in the neighborhood of 4 hours at .70 IF. If you tried to do a .90 IF TR workout for 4 hours, you simply wouldn’t finish it.
Long story short, its plenty of time. When you get closer to your event it wouldnt hurt to get out for some 3-4 hours rides, so that you body gets used to being in that riding position for that amount of time. Come April you’ll like find that you’re plenty fit, but the rest of your body will be sore.
Good luck, have fun!