It's now two weeks to race day, and ideally you will attempt a final long run this Sunday and then start tapering down, taking training more easily and generally conserving your energy. Continue to use training sessions to rehearse aspects of the race - for example, wearing in new shoes or clothing so that nothing is being worn for the first time on 22nd April.
Two points deserving attention are caffeine, and music. Caffeine is good news because it increases the breakdown of fat for energy, and so spares your glycogen and delays or prevents your hitting the Wall. Also, no drug authority minds it in moderation. I would personally take my pre-race liquid in the form of Lucozade Sport, but you could have one or two Pro Plus tablets - equivalent to one cup of coffee - an hour before the race, and one or two more in your pocket to have at half-way. Try it out this week but cut out caffeine next week, so that on race day the fat metabolising effect is boosted.
Last year I ran the marathon with a music player and it worked a treat. My thinking was: one's reaction to the Wall is a mental phenomenon - it is one's mind that decides to stop and walk - and I wanted to keep my mind occupied and contented. I chose songs with no beat, so that I would not find myself running in time with the music. They were songs that were enjoyable and somewhat stirring, such as "Barcelona" (Freddie Mercury) and "Eagle" (Abba).
If you think that music might help prevent your mind playing tricks on you, you should consider setting up a "Marathon Folder" on your music player, and trying out the songs as you run this week.