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, increasing weekly mileage by a maximum of 10% for 3 week then subtracting 20% until achieving mileage you wish is not bad. If you are increasing mileage by less than 10% then adjust according.
Stretching is important or else your body will shrivel and die and you will never contract again. Your muscles do not understand the aims of a runner and as such you must impose upon your muscles stretching to ensure they continue to carry out the task you force them to do. As long as you are not stretching until tearing or loosening your tendons or being able to bend too far in any direction you should be well enough.
