I'd be tempted to miss the marathon as I'd think you could find a similar long distance off road event and while speed is useful I'd think its more leg strength and stamina that you'll require for the Dragon's Back (...I'm no expert on this type of massive event, but have done a couple of ultra's and the Lakes Longs etc). I think they're hoping to re-schedule the Edale Skyline for instance to about the same time. I'd be aiming to race and train very specifically as you're very short of time to gain experience etc (...from my understanding from your post). Some of the LDWA events can, and are ran, and are often a gentler introduction to going long off road. I'd possibly even suggest an event like the Bullock Smith which is in September. Its 55 mile in the Peak District but infinetaly do-able even if you end up walking chunks! You'd possibly need to go up and do recce's too which would potentially focus your training. As for walking being training for running, 'time on your feet' is said to be a crucial part of training for an ultra and I'm pretty sure, if you get to do the Dragons Back, some walking will be required!