Once you begin your run/walk program you will want to maintain your flexibility. One of the best ways to do this is to stretch before and after your exercise session.
Warming up the muscle is key to getting a good stretch. Think of your muscles as elastic bands...well if you put an elastic band in the refrigerator it will not stretch well, however warm it up and it has a more elastic property. Just like that elastic band, warming up the muscle and getting some blood flow through it will help improve the flexibility of that muscle.
When you stretch you should stretch to a position of mild discomfort and then hold that position for 10-30 seconds. This should be repeated on each muscle three to four times. Also while stretching do not hold your breathe, breathe as normally as possible.
Good news, flexibility can be improved in as little a time as two sessions per week!