The Coastal Bike Trail is about 15 miles and takes you past (or through) the town of Rincon.
The Seru Largu Bike Trail is about half the distance of the Coastal Trail, but it is much steeper. Make sure you rented a mountain bike if your going to take this trail.
The Southern Bike Trail is about 25 miles, it is on paved flat roads, so it's easy going. You can check out the salt works and salt pier along the way, see Pink Beach, visit the red and white slave huts, and end up at Lac Bay.
The Lagun-CaiBike Trail is about 21 miles on flat roads, but they are dirt roads rather than paved. It ends up over at the mangrove area near Lac Bay called Cai Point. You can see Lagun, the old Spelunk light house and watch the water spray over the rocks. You may even decide to take a shower in the salt water sprays.
Bike along The East Coast Bike Trail and it will take you to Lagun during the flat part of your ride and then the trail slants upward to get to Boca Oliva. The East Coast Bike Trail is about 22 miles long.
The National Park Bike Trail is long, about 43 miles. It can be a little rough going since it is on dirt roads, as well as paved. If your going to do this trail make sure to check with the park in advance to make sure you can enter it with your bike. This trail is so long, I would recommend you do it with a tour group.