i do ironman distance tri's. this summer a buddy of mine have put together a week that includes over 30k of vertical climbing on our bikes in colorado(including 50 miles at the Leadville 50 mile mountain bike race)...If I were you I would not worry about speed but as another poster said, time in the saddle is huge. That 7 miles in 25 minutes is nearly pointless, you need to sit in and slug out time on the bike. You are engaging an 'ultra-marathon' distance for the bike...you need to emulate that event. Sit on the bike, pedal for long periods of time, toughen up your ***, there is a lot to this.
In regards to Marathon training, again the long run is key. Learn to hydrate/fuel properly. You are going to get lots of input here...but one thing most won't tell you that is a huge piece of training is sleep. Rest, rest, rest.