Mr.G Thyagarajan, a TEF member has the following to say on Back Pain  :

Best solution from personal experience is doing Bhjangasana and SuryaNamaskar.
Initially it will be difficult for Suryanamaskar but if one start practicing for more than 10 surya namskars then it will be easy and ABSOLUTE RELEIF. Surya Namskar has 12 Asanas immersed in it.
It does not stay there because then your health will be in excellent condition for your more adventure Yoga Asana, Bandhas etc

The best book in the absence of good coach or teacher is Asana Pranayama Mudra Bandha, shortly known as "APMB" by Swami Satyanandha Saraswathi of Bihar School of Yoga, Munger which has the only Yoga - University in the World.

Pure self experience and I do not have that lingering BackAche anymore when I continue Surya Namskars. One can do it leisurely in a slow mode also and it all takes mental determination for about 30 minutes that is all. We can spend 30 minutes in the early mornings or worst in the evenings but on empty stomach which will also shrink the belly in the course of time.