Your training plan for the Brighton Half probably has an expected level of performance to start the plan (e.g. currently running xx miles per week), so you want to be able to maintain that level of activity. Another useful guide is to look at the longest run in the first week of the plan and make sure you can continue to complete that distance in the interim period.
Personally, I tend to take a week or so off after the event and then spend the next few weeks running at a steady state (i.e. no speed sessions) just to keep myself ticking over.
