I totally have the feelings you are having...and dd is only 2.5, LOL!!!
I think there are specific considerations to any age difference.....the one you are likely going to have, 6+, is certainly no exception.
In psychological terms in regards to family dynamics, they say that if an age gap is more than 5 years, then you don't really have "siblings" so much as you have 2 "only children". That of course comes with its own set of issues.
From a medical/physical standpoint, they have found that when there is a gap of more than 5 years, a mother is more likely to have pregnancy complications and also that the second labor is more likely to be like another "first" labor, and not the shorter, easier "second labor" that women usually have.
Of course, these are all averages, your experience will be unique.
One thing that will likely affect your day to day life the most is that they probably won't be interested in/doing the same things well...ever.
If you do out-of-home schooling, you'll be shuttling one to elementary, one to middle, etc. Same with classes and stuff, you won't get any "two-fers" where they can both go to the same class, etc. They give discounts when you have more than one in college at a time, you will have to pay full price for both since they are so far apart. (if you believe in paying for kids college and they choose to go, etc) In terms of family vacations, etc, it will be harder to find activities which both enjoy....and of course, for the next few years, you are going to have an infant, then a toddler...so a lot of the freedom, mobility, sleep, etc that you have now will be lost for a while. That might be harder for you to adjust to than someone who is still IN that stage.....lol, I was just joking to dh that right now i feel like I've been paroled...dd sleeps through the night, has weaned herself, is potty training, and will play by herself at times......and having a baby is going to put me right back in the slammer again!! LOL!
but there are also potentisal strengths. You will have more time and energy to give the new baby, your older child can be a help, you are potentially wiser and more experienced now, and maybe in a better financial position, etc.
them's my thoughts!
good luck!