if you havent done it for a long time then DONT just jump into a 6 mile run, walk a mile, run for 3 then walk the final 2, if you just try and demand too much straight away then youll just hurt yourself, sicken yourself and end up not keeping it up.
also best advice ive read so far is invest in a good pair of trainers. if you have an android/iphone then get runkeeper to keep track of your progress.