Maybe too late for October, but in your next marathon prep try the following:
Build in sets of 4 x 800m (at marathon pace) w /100m recovery into your long runs.
2 miles moderate to track
4 x 800m @ SL marathon race pace w/100m recovery
4 - 8 miles moderate pace
4 x 800m SL marathon race pace w/100m recovery
2 miles moderate
...
2 miles moderate
4 x 800m @ SL MP - 100m rec
3 miles mooderate
4 x 800m @ SL MP - 100m rec
3 miles moderate
4 x 800m @ SL MP - 100m rec
3 miles moderat
***
60 - 90 min regular long run pace
3 x 3200m @ goal marathon pace 400m - 800m recovery
***
60 - 90 min run, regular long run pace
30 - 45 min indoor on treadmill, 0% to -1% grade goal marathon pace.
60 - 90 min run
5K - 10K "B" race at SL MP.
cool down
***
It has been my experience that most places at altitude have easy access to long declines, not long downhills - just some place that has a 1 to 2 percent grade spread out of 5 or 6 miles, find these and try to do these longer declining sections at your MP - you don't want anything steep, so that you don't beat your quads into mush. Focus on doing these longer declines at MP later in your long run - so that your legs are familiar with turning over late in the race at a good rate. It has also been my experience that my legs don't know what to do ... basically, running altitude 10K pace for 13 miles at sea level ... or half marathon altitude pace beyond 20 miles.