Pinelands Trail is located seven miles (11 km) from the southern park entrance. It's a fairly short trail, just 0.4 miles (650 m), and you walk on a pavement or boardwalk so it's easy to do. It's also handicap accessible.
I think this walk is a great way to experience this unique ecocystem of the Everglades up close. The ground is swampy but home to a forest which contains pines, saw palmettos, and assorted wildflowers -- quite a bit different than the "river of grass" which dominates this landscape.
There are many better places for alligator viewing than from this boardwalk -- to this end I recommend Shark Valley. We visited Pinelands Trail in December and there were no mosquitoes around but I expect that in the summer they might be bad here so be ready for that.