In reply to Yanis Nayu:
> Build up more slowly than you have. It takes time for your structural fitness to match your metabolic fitness (that's what I find anyway). I was plagued with shin splits when I did athletics and dearly wish I'd followed this advice.
I'd say the above is the best advice.
Getting 'fit' isn't too hard. Your CV system can make fairly rapid changes but the biomechanical elements of your body take longer to adapt, with the result that you feel like you can go faster/longer but the body starts to fall apart.
Try and run consistently - 4 short runs a week is better than two long runs.
Best of luck and have fun.