Can we explore the Polonnaruwa ruins by bicycle in one afternoon?

Yes, cycling is the absolute best way to see Polonnaruwa as the ruins are grouped closer together than in Anuradhapura. You can rent a bike for around six hundred rupees at the entrance. Start with the Quadrangle and save Gal Vihara for the late afternoon light which is beautiful for photos of the stone Buddhas. The park is mostly flat and shaded by large trees.