You can train very intense and get injured and it will be really fun because then you won't get to do anything pain-free, highly recommend.
For building mileage, a good mileage building heuristic is increasing weekly mileage by a maximum of 10% for 3 week then subtracting 20% until achieving max mileage. This can make you build mileage very slow if you are not running much more than 40 miles a week, knowing your body is a journey you must undertake and will inform you more often than not best.
Stretching is important or else your body will shrivel and die and you will never contract again.
