Everglades National Park's Long Pine Key Nature Trail is open to hikers and bikers and runs 6.1 miles from the Long Pine Key Campground to Pine Glades Lake, making a 12.2 mile round trip. The trail is absolutely flat. You will need a mountain bike or at least a good solid hybrid bike to negotiate the terrain. The trail is 6 miles from the park entrance.
Here's a map of the trails in the area...
...and here's the map posted at the trailhead.
Most of the pine trees in this area were logged prior to the park's establishment in 1947. The trail is basically an old logging road or fire road consisting of two ruts in the limestone bedrock. The pine lands are the most diverse habitat in the park -- an open south Florida slash pine forest with an understory of saw palmetto and over 200 species of subtropical plants. This habitat is also one of the last refuges for the elusive Florida panther.
There are several open sections more like a meadow than a forest, and are technically called "freshwater marl prairie." These open areas are where the "trail" is in the worst shape, with the limestone very rutted and populated by bumpy knobs of rock. I found it best to simply bike the center of the trail, staying out of the two damaged single tracks where vehicle tires would run. I wondered if perhaps the damage was due to the area being more open to the elements during storms?
Pine Glades Lake is at the far end of the trail and is a lovely location.
One last photo which includes my bike...