One of the biggest causes of back pain for newer riders is a saddle slightly too high. This has you reaching for the pedals and your hips ricking with each stroke. The rocking flexes your lower spine and requires more strength to stabilize everything.

Have a friend ride behind you and observe, and if your hips rock, drop the saddle a bit. This may be lower than ideal from a power/legs standpoint but if it relieves the pain, will allow you to ride farther. Over time the muscles that stabilize the back will strengthen, and you can bring the saddle higher without the hip rocking issues.