A C402 pilot overflew eight hours in twenty four because of airport delays after taxi out; a low; slow climbing aircraft which ATC could not issue vectors to and a longer than scheduled flight time.

Narrative

I inadvertently flew over eight hours in 24. On the last leg I flew I left the gate with enough time to complete the flight and not go over eight hours. The flight was delayed after leaving the gate for two reasons. First; there were a larger number of aircraft waiting to depart than usual; due to the weekend. Actual takeoff occurred 25 minutes after departure from the gate. Secondly; the departure requires a turn to the west after takeoff; to maintain a heading of 295 degrees. ATC kept me on that heading for a lengthy period of time before allowing me to turn toward the north; and then toward the east. The aircraft was heavy; and therefore was climbing at a reduced rate. ATC explained that they could not turn me until I reached an unstated altitude. I maintained Vy; but was still climbing at barely 500 FPM. The sum of all this was that upon arrival at the destination; I had accumulated over eight hours of flight time in the previous 24. This route should be planned for more than the current time; especially on holiday weekends in the summer; when full loads and slow climb rates could be expected.