Did this last year and really enjoyed it. The group stays together. They have a guide in the front and in the back. It is a busy road, but the guides are signaling to you so you know to stay on the side. You are using the brakes most of the time with only a couple of times to uphill (a short hill) pedal. They stop frequently to make sure everyone is ok and see the area. You really should be keeping your eyes on the road while riding. You ride close to each other so the group stays together. I second going through customs on a bicycle is fun and probably something most people don't get the opportunity to do. The day we did took this excursion, it was really foggy when we got off the train so they took us a couple miles down the mountain to be out of the fog. Helmets are required and provided.
Before we went, I saw a review from someone who was disappointed that they couldn't ride at their own speed as they were avid bikers. It doesn't sound like this fits the original poster, just mentioning it for anyone else reading this. You stay in line, no passing, no riding ahead (or behind) the guides.