I have run for many years and I have naturally poor flexibility.
I have started yoga and this has improved my flexibility. If you can’t afford regular yoga classes buy a few basic yoga DVD’s and practice at home all you need is a yoga mat, which are very cheap. I do the latter and have never attended a class.
I have received benefit from doing yoga and it did stop the lower back which I had for number of years.
If you have any discomfort from doing yoga please consult a medical specialist.