The upper section of the Petawawa is out in the boonies, but worth it if there is a good flow in the river. The whole undertaking involves lots of driving and possibly some walking, and the paddle is suitably rewarding. The riverbed has been modified in places by loggers, so take care if you are a regular swimmer. In high water, it is worth walking up from the take out on river right to scout the last rapid. If this is looking a bit fierce for you, there is a smaller, easier channel through the woods on river left. Other than that, there are no surprises. In lower water there might be a portage around a small drop with a shallow landing. The river is used by canoe trippers, so there are portage trails everywhere.
To get to the take out; First check that you have enough gas for 160 km of driving! Then from highway 17, turn south onto Doran road (about 6 km south-west of Petawawa). Almost immediately, turn right onto Barron Canyon road. This leads to the Sand lake gate to Algonquin Park. Stop and pay your day user fee, and maybe ask about the road conditions. Continue on Barron Canyon road, which turns into Lake Travers road. The take out is after the 71 km marker, at the bridge over the Petawawa as it enters lake Travers. The park map here might be useful.
To get to the put in; Retrace your route back from lake Travers for about 1.5 km and turn right. Follow this road for 3 km until an old rail bed is reached. Follow this to the right until it is blocked by boulders (a narrow vehicle might fit through!). Put your boat on your shoulder and start walking. You will cross the river on a wooden trestle bridge, and after that put in wherever is convenient. The further you walk, the more river you get to paddle.