You haven't really mentioned if you plan to stay overnight anywhere, which is a lot different than taking day trips.

I didn't think I'd need anything more than a saddle bag until I did my first overnight bike trip. It wasn't even a lot of mileage - maybe 40 miles or so - but I ended up in a different state, in a place that had a lot of touristy stuff going on. Staying overnight got me so hooked that, the day I got home, I went out and bought a fishing tackle bag to replace my backpack and have just kept adding storage capacity to my rear rack since then.

Now I use a Topeak MTX rack trunk/pannier combo set with the matching quick-release rack and I can carry every tool, every spare part, and every item of clothing I might need when I do an overnight trip. It makes the ride that much more relaxing, because, until you've torn a sidewall, broken a spoke, or snapped your chain out on the road, you don't know what a pain that can be if you don't have the resources to fix it!