300 calories every 30 minutes sounds like a lot, to me. It does depend on your body type, though. I like plain water, gel, stinger chews, stinger waffles and enduralyte tablets. Water, because it is easy to find. Chews, gels and waffles because they are easy to transport (and taste good.)
I will also bring a long some salty snacks and nuts to mix things up.
A lot of people like Hammer and Infinit Nutrition. They are worth looking into.
Really depends on your taste and what you can keep down. Everyone is a bit different and the only way you'll find what works for you is trial and error. I like solid food and am ok with gels, I have a teammate that only does liquid. Experiment a bit in training and find what you like. When you find something you like stick with it through the event.
With the disclaimer out of the way, 600/hr sounds like too much. I've had some luck with "take your bodyweight and double it" method. So if you weigh 150lbs, the max you would want to try and put down in an hour is 300 calories.
Hydration is going to be the most important thing. Keep sipping every 15-20min.
There are three things you need to be concerned about on a 150 miler in terms of nutrition: hydration, calories, and electrolytes. Hydration is important for obvious reasons, and on average you'll want to consume about 24oz. of water/hour.
For calories, you'll be expending about 500-600 calories/hour, but your body can physiologically only process between 250-300 calories/hour, so make sure you are ingesting only this much per hour (do your computation by figuring out how many hours you'll be out and multiplying by 250 which will give you the total amount of calories you should plan on taking with you). The rest of your caloric intake will be through your glycogen and fat stores. So, let's say that you plan on doing the 150 miles in 10 hours, then that means you'll need to consume 2500-3000 calories (10*250/300) for the ride. I would not go above this amount (250-300/calories/hour) or you might end up with GI issues due to the backlog at the stomach.
Finally, electrolytes are the last bit of the equation. Your sweat contains sodium (0.9 gram/liter), potassium (0.2 g/l), calcium (0.015 g/l), magnesium (0.0013 g/l). Make sure that you are drinking enough electrolytes. By the way, most folks think that only role electrolytes play are in preventing cramping. But the primary reason to include them is that sodium activates the glucose transport mechanism which causes an increase in carbohydrate and hydration absorption rates.
Finally, a lot of folks overlook the importance of taste. You're going to be consuming your fuel for 10 hours...so make sure you like the taste!
Oh, one final note...as stevland said above, EXPERIMENT. Everyone *is* different so try out your fuel strategy before the 150 miler - preferably on a 5 hour ride. Nutrition problems are compounded the longer you go...