I would have to agree with AliceW...
It does not matter how often I ride, it will still hurt the day after. I guess the stronger I feel, the harder I ride
I mostly get sore shoulders/arms/hands
However, lower back pains tend to come from a bad posture... you probably ride with your ass pointing too much behind you... I don't know if you see what I means : you have to reverse your hips forward and keep your ass as close as possible to a line made by your feet and shoulder...
I hope it helps...