I think you can. As a matter of fact, if none of your competitors have juiced fasted, you could probably win. You are far into your juice fast which means you have done a tremendous amount of detox already and the rebalancing of your body is in full swing if not coming down from its peak. The less detox you have to go through, the less balancing, building and healing your body has to do, the more energy you will have. I imagine your energy levels and stamina levels are stunning about now. I know mine were at Day 34 (stopped the fast then).
There are preparations you can do to enhance the marathon such as continue drinking those juices, focusing on the greens, especially wheatgrass... meditate regularly along with prayer. I've heard that some runners can go into a meditative trance while running.
Would it be possible to have friends and or family stationed at specific intervals, say, every five miles, with fresh, raw juices, along with water ready for you to drink, while you are racing?