Pip. 12 miles will be fine. There is no intensity to it. Just take it nice and steady. It is not as if it is a 12 mile intensive run. You will have plenty of time to recover from that , esp as it is easy, not fast, and if you have a nice half marathon taper over the week you will be fine. not quite the same as doing a 20 miler and then having to taper for a marathon. What half are you doing ?