First, if you’re not convinced that athleticism really matters in driving, you can take a look at this chart of my DAQ and heart rate from Laguna Seca:
http://www.ghidinelli.com/2009/09/20/fitness-in-auto-racing-matters
One of the books I have is this one:
http://www.amazon.com/Motorsport-Fitness-Manual-performance-physical/dp/1844255433
I had the fortune to spend a weekend with the two authors and Mark Webber at Silverstone last January at the Porsche Human Performance center. There’s really good stuff in there. Unless you can afford a private trainer, your best bet is something like Crossfit which combines strength training (weights) and anaerobic/high-intensity exercise with some coaching and camaraderie. What most people overlook is that lifting weights builds muscle which burns calories 24x7 and that helps maintain your weight. It also makes you stronger! And with high-intensity training, there is a growing body of evidence that you can train much less but reap just as much, if not more, benefits from the effort. Obviously you don’t need to deadlift 500# to drive a racecar but if you’re doing your own wrenching, having good technique lifting heavy things keeps your back in good shape and makes your overall weekend at the track easier.
While there may be benefits to eating less frequently as a lifestyle, on a race weekend the last thing you would want to do is eat one big meal a day. When you eat, your body redirects energy and resources to digesting food. While you can mitigate the blood sugar impact (and thus, the way you feel and perform) by adjusting what you eat (look for foods that are low on the glycemic index), you want to eat and drink before you’re hungry or thirsty and in lots of small increments (like 6 meals a day if you can manage it). Most of us are on track 2-4 times per day on a race weekend so you want to minimize the impact of food and drink. Eat and drink healthy, but do it in lots of little bits. It’s true the carb-loading is sort of old sports science at this point. In fact, even hydration is being rethought. One of the things I saw at Silverstone last year was a chart of hydration level vs. marathon times and the fastest marathonners in the world are actually performing a bit dehydrated. There is speculation that this comes from our ancient history as persistence hunters where we didn’t have ready access to water all day while chasing down gazelle.
Anyways, here’s some great resources for more info in addition to the book above:
Jim Leo from PitFit Indy has e-training programs you might look into. You can also read some Q&A with him from SafeIsFast here: http://safeisfast.com/qa_sessions/5
I have a youtube channel of racing + fitness related videos. Some of them are semi-puff pieces on F1 drivers, but others are quite good: http://www.youtube.com/playlist?list=PL011C8776043E6839
And finally, for most weekend warriors, one of the best things you can do is put a cool suit in your car. You might think you don’t need it, but it reduces dehydration and it keeps your core body temperature stable. There are stats that I don’t recall off-hand but something like even a 1 degree increase in body temperature leads to a certain % of concentration loss. Do you need to concentrate in the car? 
You can build one on the cheap with these directions: http://www.ghidinelli.com/2011/02/28/diy-cool-shirt-system
And at the end of the day, whether it improves your racing or not, being fit and strong will help you in every aspect of your life. So it’s win-win-win.
