If you are committing to indoor cycling, then commit to investing in cycling shoes.  You will be more comfortable and get better results with clip-ins.

The stiff soles on cycling shoes keep your feet flat while you pedal (sneakers, which are flexible, can cause you to point your toes, slowing you down and making it difficult to effectively pull up on the pedal to complete the rotation).  In addition, cycling shoes don’t have a lot of cushion, which means there’s nothing to absorb (or stop) the energy generated from your core down.  This helps transfer more power to your pedal stroke, so you are able to move faster during intervals and build stronger uphill climbs (which translates to more calories burned).  Also, the ball of your foot should be centered on the top of the pedal.  In a sneaker, you are more apt to slip around on the pedal.  Spinning cleats clip to the pedal, locking you into that sweet spot and ensuring that you push down and pull up throughout the entire circle – engaging more muscles!