>>2044685
The answer to your question is really 'everything.' Fitness (aerobic capacity, strength anaerobic capacity, flexibility), technique (smooth cadence, knowing how to pace yourself, practicing eating/drinking on the bike, how to dress), and bike setup (appropriate fit, mechanical efficiency, rolling resistance, aerodynamics) all influence what kind of speed you can sustain.

Biggest thing you'll want to look up is the concept of interval training.