I think the secret to returning to running after injury is to take it easy and not to let ego, goals or overenthusiasm cloud your judgment! I think you need to decide whether being able to go running is more important than entering an event. If you make your goal being able to be fit and go running and entering events is an added extra that you do for fun, but is not the primary goal, then the decision to ease back and not stick ridgidly to a training program is much easier when a niggle appears. If you listen to your body and not your ego, then taking a couple of weeks easy will mean that you can then continue training normally when you feel happy. If you ignore the niggle and it develops into an injury, then you may well find you have to take weeks or months off and you will be starting from scratch.
If the injury is recurring try something different, for me using minimalist shoes did the trick and allowed me to run regularly again. That may or may not do the trick for you, but if you are getting recurring injuries something is not right, experiment with shoes and technique until you find something that works.
When you start training again take it really easy, really easy! When I came back to running I took it easy for 3 months, then in my elation at being able to run again I overdid it and tore my soleus. I had to take 4 months off to get over that, annoying, but it tought me a really good lesson. After that I started running for 10 minutes 3 times a week. I think that it is really important to use time not distance as the variation in terrain I run on means that my pace can vary from 8 to 13 min mile. I increaced my run by a minute a week until I reached 40 minutes. I kept my evening runs at that and increaced my weekend run by 10% per week. Just before Christmas I was up to 2.5 hours for my long run with no injuries. I was then struck down by the lurgy and did not run for 4 weeks. I am now fully recovered from that and have resumed my running. I have dropped my weekend run back to 1 hour and am 2 weeks into adding 10% a week.
I have a list of events as long as my arm which I would like to enter, but am not going to do so until I am certain that I will be able to do them without doing my self in. I would rather be able to run every week than mess my self up trying to acheive something which I am not yet capable of.
Sorry about the long post, but that is what I have found works for me.