Unfortunately it is inevitable that, with so many runners, some people will get injured in training for a race. What happens next depends on how you got your place and what race you are doing.
If you are using your 'own place' to run for Oxfam (i.e. you obtained it directly from the organisers of that race) you simply need to let them know you can no longer take part. They will decide whether you can defer your place to the following year. We'd also appreciate it if you could let us know so we know how many runners to expect on the day.
If you have a guaranteed charity place, which you obtained by paying a registration fee with Oxfam, then you must let us know as soon as possible if you are unable to take part in your event. We cannot automatically defer your place to next year's event, and all registration fees are non-refundable.