While most electric races are 10-15min I am used to those 20-30min mixed Elec/gas mains your speaking of. If you dont mind a battery change get those 4500mah hardcase that gnr mentioned.
A battery change can be done in under 30 seconds which will be about equal to all the gas pitting the nitros do in a 20-30minute main. So they shouldnt complain when you point that out.
The only reliable way to get 30min nonstop with hard racing throttle is probably 6cell 4500mah-5000mah which will be a huge heavy pack. I like 6cell cuz it runs cooler especially in the summer ( a 4cell 5000mah= 6cell 3200mah roughly for run time).
See
http://www.oople.com/forums/showthread.php?t=37230 for more details on that.