I could have written this message about 18 months ago when I found out I was expecting twins for my 3rd pregnancy (with a 2 and 4 year old already). In fact, I pretty much did.
I was in complete shock and despair about the whole situation, thought (like you) that everyone would think we were freaks, had no idea how we were going to cope, just felt so sorry for DS 1 and 2 that we had brought this situation upon them and the family.
Well, my twins are now 1, and I wanted to reassure you that it has all been so much better than expected! The first 4 or so months were crazy and we just got through them, but since then it has been getting better and better. When you are pregnant, you just forget how lovely young babies are , and how they wheedle their way into your affections! And before long you pretty much forget that your family was any other way.
That said, it will be hard work. Any help you can get will make things so much easier. Call Homestart and see if you can get a volunteer one morning/afternoon a week. Maybe see if you can get a childcare student from the local FE college or something like it to come and spend a few hours with you in the afternoons (for minimal pay) to get work experience credits.
Also, if anyone can take your oldest two(?) to school in the mornings(or at least some mornings), that will be a huge, huge help.
Good luck. You will adjust, and there will come a time when you walk out of the house with all 5 with your head held high, the "you've got your work cut out for you" type comments will bounce off you, and you may even feel sorry for all those "normal sized" families with 2 or 3 kids who don't seem to be having nearly as much fun as yours...