I have been doing weight-lifting and floor exercises with a focus on building abdominal and upper body strength.  It really does seem to make a difference, especially with pushing through hills and endurance for the LR schedule.  Everyone is different, but it seems you have wonderful access to a variety of equipment, so I would target the core muscles, as BOSNPM suggested and then try to identify any weaker areas that might be assisted with other exercises.  We all have those situations, so determine any problem issues where you could benefit with greater flexibility, strength or stability. 

Wishing you all the best for your Marathon!

I am 6 weeks into a 100 mile training program.  During this time I have started to incorporate Insanity workouts 4 to 5 times a week.  These workouts have been total body workouts but I definitely notice how weak I was in my gluteus and hip region.  The program involves a lot of high intensity intervals.  This allows an increase in your VO2 max and also your lactate threshold which help you with your stamina and your top end speed.  I have been able to run faster while holding my heart rate lower.  I would recommend any type of exercise that works your core, hips, and gluteus.  I was really amazed how increasing my strength in lateral movements has helped my running form.  If you can't do the insanity or a p90x I would look into different tomata workouts which are used to really get your heart rate up.

I'm boring. I own an elliptical crosstrainer and an exercise bike, and those are the bulk of my crosstraining. I also use free weights and do core work twice a week. Once each week or 10 days I get to the gym and use other crosstraining machines (usually the Stairmaster or a rowing machine) for variety.

For a runner, I do an unusually high percentage of aerobic crosstraining, about 50%. I can only run 2 or 3 times per week because of a history of knee problems (congenital misalignment and instability). I don't consider it cheating because I don't have a choice. If I tried to run 5 or 6 times a week, I would have wiped out my knees years ago. As it turns out, I am now 56 and still happily running. Many of my running friends who didn't do any crosstraining have long since burned out or turned to other sports.