I rode from Maine to Louisville, KC to Fayetteville, NC and back to Boston in July in 2011. Bike was showing upper 90s at highway speed. All of these are good suggestions, but you need to figure out what makes the trip more comfortable for you. There are plenty of WalMart's along the way if you need to make gear adjustments.
-If you are going to spend any time riding at night, highway or back roads, definitely invest in some lights. You can get a set of Sanjoh's LEDs for around $100. That was literally the first thing I did when I got home after critter dodging in NC at night. Even if you can only spare the cash for 1, it will make a world of difference.
-I only wear Ex-Officio boxer briefs and t shirts now. They are spendy and there are alternatives, but anything synthetic will take the edge off the swamp ass/back.
-Don't have personal experience with this one, but know plenty of guys who fill the camebak with ice water to keep them cool and have constant access to water.
-Make the seat comfortable. Even if its lashing down some foam or the notorious $20 WalMart ATV seat cover, a sore ass makes for short days.
In terms of traveling, I found the OK tolls to be a pain in the ass; exact change, no toll workers, and expensive. Radar detectors are illegal in VA and the LEOs are always out in full force, so I avoid it when I can. WV is epic, I'd pound tar to get there so I could have some extra time to go though Monongahela National Forest. PA highways > MD and DC by a long shot. Not much to look at, but you'll keep the tires moving, which is especially important in summer. The OP didn't mention camping or hotels, but for a short blast like this where time is a factor it is a lot less stressful to pull into a motel and not have to worry about setting up camp, etc.
Hope this wasn't too long winded