My b/g twins are 14 months old. My pregnancy went perfectly until I was diagnosed with severe preeclampsia at 32 weeks (I taught full time until 31 weeks). My kids were 3lbs 15 oz and 5 pounds and spent five weeks in the NICU (mostly because they needed to learn the suck/swallow/breath thing). My advice to you would be:
Have a schedule, stick to the schedule, and keep the twins on the same schedule. I kept my kids on the same schedule they were on when they were in the NICU. At first, they ate every three hours, but were on demand at night (when one kid woke up, we woke up the other so they ate together). After awhile they changed to an every four hours schedule. By having a schedule you can plan your life (I would try to get one of the twins nap to correspond with your 1 year old' nap so you can have a break) - this is especially good for getting people to come over to help (definitely get help!). It is much easier for them to help when you can give them exact times that you'll need help. I had someone come over and help with one feeding each week day for the first four months and it was wonderful! I have some friends who had three kids before their twins came along - they didn't keep the twins on the same schedule and it was a nightmare. They refer to that first year as "the dark year." A schedule makes things so much more doable.
If you use bottles, have at least enough for one day so you don't have to wash bottles all day.
I highly recommend the Combi Twinsavvy stroller. It's amazing - turns on a dime, folds up to the size of a golf bag, is no wider than a wheelchair so you can go anywhere that's handicap accessible, reclines, has cup holders, the underneath baskets are sufficient, the front bar comes off so it can be used as they get older - I have another friend with twins who used this stroller for the entire time her kids needed a stroller, so it's useful for all ages and is durable. The only downside is that you can't attach two infant seats to it. It means you have to take the kids out of their seats and put them into their stroller. It wasn't so bad for us and it was very worth it since all of the other features are so great. Besides, your kids won't be in infant seats forever and how much are you going to go out with three little ones anyway? :)
Mentally prepare yourself for bed rest, a c-section, and your kids spending time in the NICU. You might not have any of these things, but it's better to be prepared than not. We found our NICU experience to be a very positive one. We knew our kids were going to go to the NICU so we weren't surprised when it happened. I would say the only days that were hard were: the day I came home and they didn't; and the day my daughter came home and my son didn't.
As for the pregnancy: Yes, you'll be huge - you're carrying two kids. Yes, it'll probably be very different from your singleton pregnancy. I've only had twins, but everyone that I've ever talked to that has twins and singletons says it's very different. I taught full time, but sat as much as I could when teaching and took it easy at home. That won't be easy with a one year old so you might need to get help before the twins come so you can rest and keep those kids in you as long as possible.
I recommend you get the book "When You're Expecting Twins, Triplets, or Quads." It has a TON of useful information, including how to help you through your pregnancy and help it be a healthy one.
Don't be afraid, this is so exciting! You have two little lives in you. Yes, twins are hard (especially in the beginning), but it gets so much better. Yes, it's a high risk pregnancy, but you'll see your doctor a lot more than you did with your last pregnancy - trust your doctor, take it as easy as possible, and eat a lot!
Good luck!