Sandi, my tip is to quit worrying! You are going to have a wonderful time on this trip. The scenery and historical sites are awesome, the food is good, and the whole experience will be a fun one for you. You will enjoy the company of your fellow travelers; the first few days try to sit with a variety of people at meals so you can get to know them. There will be many olther single travelers, some of them first timers. They will be just as anxious to meet you as you are to meet them, so go up and introduce yourself. ( And you will enjoy the Europeans you meet, too!) The only difficult part is getting there; the long flight and jet lag are just the price you pay for seeing the world.
Take your most comfortable walking shoes, for there is a lot of walking and there are many cobblestones. Sunscreen, insect repellent, windbreaker, hat, sunglasses, small umbrella and/or poncho or raincoat. A bag (purse or tote) with a secure closing top to carry around each day, big enough for a small water bottle. A good camera you have practiced with, extra batteries, and plenty of memory. Whatever medications you need, of course. Keep your money secure, probably in a money belt or neck pouch, and put it in two or three different places on your body and in your bag or carry-on, never in your checked luggage. Take a good general credit card, let your bank know where you are going and your dates of travel.
We took all but onel of the optional tours, and didn't regret a single one of them. Research on-line the different museums in Amsterdam before you go, in case there are any really special current exhibitions that you especially want to see. We didn't think the optional trip to the Rijksmuseum and Van Gogh museums spent nearly enough time, so we didn't take that extra option.. Near the boat dock there is an inexpensive shuttle which goes right to the Hermitage Museum (as well as many other places), and you can, from there, walk the long, pleasant walk to the Rijksmuseum, or take a taxi. The Van Gogh museum is right at the other end of the grassy park. You can manage to go nicely through two of these three museums in the time you have, or do one of them very thoroughly. You could also go to one, and then spend time seeing some other sight, such as the Anne Frank House or Rembrandt's house. Do not be afraid to take a signed taxi "home" at the end of your sightseeing day; as I recall the cost was $10 or so, well worth it if you are tired or your feet hurt, and taxis in Amsterdam are regulated and safe.
If you are interested in Roman History, do not miss the outstanding Roman museum next to the Cologne Cathedral.
Each stop is different; some places you are in large cities, and other times you are in smaller towns or villages.Sometimes you run across something unexpected, like a flea market, or a church fair. The river is peaceful and beautiful, the program directors are outstanding. On the guided morning tours, they present so much interesting information about the places you are visiting, and since you are a teacher, I know that you will like that. Then you can spend the afternoons exploring on your own, or with the new friends you will meet.
Have a great trip!