Well, it sounds like you have your priorities in order!
As has been said, it's do-able in 90 days but you'll need to keep moving. On a trip of this scope, I suggest having weekly goals instead of daily goals. Meaning, break your trip down by the week with an idea of where you need to be at the end of each week to not get too far behind schedule causing you to rush at the end. You also don't want to find that you rushed the first part of your trip, zipping past things of interest, fearing you would run out of time and then have more time on your hands at the end of your trip. So figuring out the pace you want to set is handy.
Also, at least once a week, it's nice to have at least one day where you don't drive much, if at all. Two is better. While I love to drive, it is nice to get a break from it. Especially with the length of trip you're taking. Obviously, you'll want to plan these for places that have many places you want to visit close-by.
I think you're on track to have a great trip and you seem to have just the right attitude about it, too. Let us know how we can help you further.