Just before the downhill to the Rincon beach turn left to the dirt road which will lead you to this heavenly jungle riding. This is one of the sections of much bigger Caminos de Osa hiking, biking trail system.
It's often muddy but just keep pushing forward, it will pay off.
Once you reach Corcovado Lodge, trail becomes narrow and rough so biking is hard or impossible.
I went in for 30m and gave up. You could leave the bike at the lodge and continue hiking.
Bring a lot of water and food, there is nothing available.
Enjoy and respect!