Thanks for writing in to the forum with your question. Give that you live in what we call a “terrain challenged” place you really do need to rely on the that gym based ME progression when you have a goal event like yours. Since you have the dam I would recommend doing 1x/week of the ME program and 1x/week of hill sprints on the dam since it is probably relatively short and steep. You can read about hill sprint detail in the book. This is a combo I use a lot with runners I coach and it has proven very effective. There is no more specific strength/power training for runners than hill sprints. I have seen the benefits of the ME training block continue to show gains for about as long as the block of training lasted. I typically use the ME until about 6-8 weeks before the race and then switch to uphill intervals for 4-6 weeks. These will help maintain the ME gains but give some needed cardio vascular benefit. These can be done on a treadmill or better yet incline trainer. Then a 2 week taper.
I hope this helps.