Jeffrey,
My attitude is there is no 'good' time to start training, you've just got to go for it and work your hardest.
Regarding the problem with your back. I am not medical expert, I would go and see a Aviation Medical Examiner (AME). I have a class one medical and through this experience I suspect that you may have a problem getting a medical.
For your initial class one medical you have to go to Gatwick, and they charge £400 for it. Maybe phone them at Gatwick and take it from there.
Aeromedical Centre
Aviation House
Gatwick Airport South Area
Gatwick
West Sussex RH6 0YR
01293 573700
Hope this helps.