There's no way you get 30-35 minutes continuous running. There's a lot of stopping and going there. To run 30 minutes on 3000mAh batteries, you would have to average just 6 amps. The Titans pull about 8-9 amps each at max efficiency so that's 16-18 amps total. That looks like about 12 minutes of continuos running to me. Not saying that you can't go out an play for that long, but there are many variables when it comes to runtime.

There's some nice C cells out there (3600mAh), but I'm not aware of any D cells that can't hold up to high current drains. You gotta watch motor and esc heat too. These systems were designed with 3000mAh capacities in mind.