This is from a July 2001 Algonquin park trip. We spent two days in the park; hiking 15km each day. The mosquitoes were not too bad and there were very few black flies and deer flies.
The hiking in the park is hilly for Ontario, but it does not compare to the mountains. The trails are not always flat and smooth. Often there are rocks and roots to step over. The scenery is often just hills and hills of trees with only lakes, sky and rock breaking up the view.