Alan
A great achievement (don't you just hate the odd minute!).
Almost everyone on this forum will have experienced that 'post-acievement lull'. It's totally natural but it is temporary - unless you let it overcome you, but you won't will you.
Think back to last Sunday:
what a buzz, 47000 people all sharing a common goal and experience, you running over 13 miles, all those people you beat, the cheers from the crowd, the thought of what a difference you were making to the lives of people you'll never meet by raising over £600, and then the finishing line, lifting your head up to the clock, breasting the tape (even though it had long since been taken down), clutching your medal, how huge your chest felt with the medal on it - no reason at all why you can't keep that forever and repaeat it.
Can you find another race in the diary, maybe a 10k, to give yourself a new goal. As for training some people swear by the RW programmes others (I confess I'm one of them) prefer to develop a routine for themselves - the important thing is to give yourself some sense of routine targets (eg x miles a month, going out y times a week/month) that you feel comfortable you can stick with -especially as the clocks are about to change & the nights will be dark before lots of us get home from work.
Hope some of this ramble helps
Keep posting