Park Runs are informal timed 5k running events held at over 50 locations in the UK. It's free to enter and whilst everyone runs together the vibe is more one of racing against yourself than others. Your times are recorded and logged so that the website can track whether you get a pb, etc. I was slow - 21.06 (when I was "fit" at the beginning of 2010 I did the second half of a 10k in about 19.30) but it was great to open up a bit after just over a month of gentle plodding. I'm planning to do one of these Park Runs each month as a way of tracking progress with the hope to get down towards 19 minutes by mid summer. The priority however is slowly-slowly building up to make sure I avoid yet more injuries.
If you do any running at all, from racing to moderate jogging I would highly recommend checking out your local Park Run. At the Sheffield event there were some who finished in the low 17s (low 16s in previous events) but also others closer to an hour, so its the sort of low pressure event open to everyone. More details here http://parkrun.com