Hi Stephen, we represent an osteopath who sees a lot of runners, so can hopefully give you a few useful pointers.
As previous posters have correctly pointed out, good posture is essential. Poor posture in everyday life places tension on muscles, ligaments and particularly discs. If you put your back under this pressure on a day-to-day basis through poor posture, then your body is less able to accommodate increased physical demands which can lead to back pain/injuries when you run run.
You mentioned that you experienced pain after running further than you have before, so it certainly sounds like this increase in activity coupled with poor posture could have contributed to your pain. If your discs haven’t been strained through poor posture in everyday life then they will be healthier and more absorbing when running which, as you know is an incredibly repetitive and high impact sport.
Good posture both day-to-day and when running can help to strengthen your back rather than weaken it, but consciously changing your posture can be really difficult after years of moving the way that you do. This has led Adam, the osteopath, to invent a product called PosturePlast (www.postureplast.co.uk) which is an adhesive support for the lower back.
By keeping your back in its 'safe zone', it promotes good posture on a day to day basis, keeping your back healthy and making it more able to absorb the impact of running. PosturePlast can also be worn during running/exercise so keep your back in the correct position during these activities.
PosturePlast works to help retrain your posture in the long term, so may be worth giving a go if you are still suffering.
Hope this helps and if you have any questions let me know!
Edited: 16/02/2012 at 13:09