The Rickenbacker Trail runs across the Rickenbacker Causeway and down the length of Key Biscayne. It begins at the Causeway tolls. You can park on the mainland side near Alice Wainwright Park, or across the on the other side of the tolls in the beach parking. We parked on the mainland side.
After blading past the tolls on the bike path, the trail leads through the parking lots and past the beach until it turns onto the bike path/sidewalk for the big bridge. Blading up 150' for so is hard work, but the view at the top of downtown Miami and the bay is killer. Unfortunately, you have to control your speed down the other side because the expansion joints are open, and you have to step across each one to avoid catching a wheel.
The trail continues across keys and bridges with a variety of pavement quality until it lands on Key Biscayne island. There, the trail becomes a bike lane on the side of a divided four lane highway. The cars are loud and close, but the pavement is fine.
In another mile or so, you enter the town of Key Biscayne. You can follow the bike lane straight through town, but there's not much fun to that. We took the first right on Harbor Drive and circled the town - much better! Nice sidewalks and roads and little traffic. Watch the intersections on Key Biscayne, though. They are brick, with lips around the rotaries. Lots of little gotcha's....
After skating back out to the main road along Island Drive and Knollwood Dr, we took a break for lunch in town. Then, right again on the main drag. A couple hundred yards bring you into the Bill Baggs State Park, and the skating gets much better. Wide paths, nice trees and views, few cars. You have to stop on the way in to pay $2 per person. We did a few laps and stopped at one of the beaches - great stuff!
On the way back out, we stopped on Virginia Key and did a little loop into Virginia Key Park. Even though the beaches were officially closed, we could wade around and enjoy the quiet and the sun.
Finally, a last push across the causeways and we were back. A great day - if you go straight down and back, it's 19 miles. With the loops on Harbor Drive and around the Baggs park, maybe 25. As we did it, around 30.