Hi congratulations!!
I had a charity place last year for Guys and St Thomas Kidney Research (my cousin had the first living donor from a friend last year).
It was a drunken "i'll do it with you"!!!at my wedding that led me to 26+miles last April.
I must admit that although I am a little overweight I could quite happily run 5-7 miles without thinking about it too much, but 26 miles is a different matter altogether.
I too celebrated my 40th birthday in the January.... i can tick it off my "must do" things now!!!!
I am by nature not built to be a sprinter,so I just took it at my normal training pace - especially at the begining when it is VERY tempting to run as fast as everyone else is. DONT do it - do your own thing - you will be very grateful at 18 miles that you did. (I am sure that is one of the reasons I didnt hit the preverbial wall as they say.)
Get two decent pairs of trainers made for running - go to a specialist sports shop rather than one that does all the trendy gear - you should be able to buy a pair for around £60ish - there is no need to spend a fortune on them (if they can do a "gait analysis" they will advise you on a pair that will support you for the way you run,rather than just general support.) DO NOT WEAR FASHION TRAINERS!!!!
Treat yourself to some nice training kit - it will make you more inclined to put in on, rather than old worn out stuff. Again it doesnt have to be expensive, just fit for purpose.
MY TOP TIP ------ put your favourite music on your Ipod and just run (I have an ipod shuffle which fits just nicely in a little pocket in the back of my trousers and doesnt weigh anything at all.
SECOND TOP TIP - put your name on your running t-shirt on the day- it really really really helps when people cheer you on by calling out your name.
(If you are well endowed put a large lump of vaseline under your endowements - will stop the skin rubbing)
The crowds are just fabulous all the way around- they will hand out jelly babies and jaffa cakes. I found that the atmosphere was so great i didnt run with my ipod - which I couldnt do without in training. I found it rather sad that people missed out on the atmosphere by running with their earphones it.
THIRD TOP TIP --- use pub/McDonalds toilets en route - it will add ages to your time if you stop to use the portaloos!!! (I wasnt too bothered about my time, but when I realised I could have been over 45minutes quicker I was most annoyed!)
You will need to put the miles in in training, but you dont have to stick to a published shedule if you dont feel like it. If you feel like running more, then do it - if you dont feel like it have the day off and you'll be more inclined to do it again the next day. Listen to your body - it will hurt (but shouldnt do too much - more achey than painful), but no pain no 26 miles gained!!
ABOVE ALL - enjoy it, you'll be more likely to stick at it
Good luck and if I can be of any help let me know.
Katie x
Congratulations, best of Luck! 
