Little bouts of stretching pre- and post-run will help you on individual runs, but they’re more about maintaining flexibility than improving it. For the latter, you need a few longer sessions of 15–20 minutes each, a few times a week. If you can do these in the two hours before a run, great.
Otherwise, find a few blocks during the week for this essential insurance payment. If you don’t run every day, then your non-running days are an obvious opportunity. Another easily claimed block is a weekend afternoon, especially if you’ve run long in the morning. Early in the morning on a workday, or midday if you can find a suitable location, are also times when others might leave you along long enough for you to get something good done for yourself. Finally, a brief session soon before going to bed when the rest of your responsibilities for the day are taken care of is a nice way to unwind and end the day.
And bear in mind that you can sneak in little bouts of stretching throughout your day. You can do foot circles (described below) sitting at your desk. You can take a short break and instead of listen to someone blab about what they watched on TV last night, loosen your shoulders and neck (also described below). Continually stretching during these “stolen” times will help keep you loose enough that more dramatic intervention, in the form of several long stretching sessions each week, won’t be as necessary.