Generating dense, valid 3D microstructures for curved-fiber composites with high aspect-ratio fibers (L/D > 100) remains difficult due to geometric blocking and early jamming. Traditional RSA methods stall at low fiber volume fractions, while optimization-heavy alternatives remain computationally expensive with limited gains. We propose a robust two-stage generator for long, strongly curved fibers (L/D = 150): (i) Constructive Self-Avoiding Walk (CSAW), which builds initial spline fibers using twist-free Bishop frames and a biased walk to control orientation; and (ii) Compress and Fill, a non-destructive densification process that combines centripetal compression and local curvature nudging to create voids that are immediately refilled by new CSAW insertions. The method reaches Vf ≈ 48% in about 5 minutes, substantially outperforming existing approaches while preserving strictly collision-free geometries suitable for FE/FFT simulations. To demonstrate mechanical relevance, we compute the homogenized stiffness tensor Chom at contrast 100 using a high-order gradient formulation. This framework provides an efficient path from realistic curved-fiber morphology generation to reliable multiscale property prediction.