It seems to me you know what you’re doing. Your program combining heavy bike mileage with selected, quality runs should whip you into very good shape. And the decision to move from the flat, paved bike path to the hilly dirt trail for the sake of some boredom-busting variety was also a good one. We must not undervalue enjoyment in our training.
Your question also reveals experience-based wisdom. You’ve identified the right considerations to weigh against each other. What you’re asking essentially is whether the pace of your long runs is more important than your effort or vice versa.
The answer is that they are both important, and neither really trumps the other. It would be a mistake to make too great an effort too often in your long runs for the sake of hitting your goal pace of 10:30 per mile. That’s a recipe for burnout. On the other hand, it’s unlikely that you will be able to sustain your goal pace in your marathon if you do not run a fair number of miles at that pace in training.
The crux of the challenge facing you is that hills, a dirt surface, and summer heat make it harder for you to sustain your marathon pace (or any other pace) compared to when you’re running on that flat, paved bike trail in the cool of spring. Again, I advise that you do not try too hard to run 10-minute miles on those dirt hills in the heat. Instead, do most of your long runs there at an effort level that feels equivalent to the effort you would normally put out in long runs undertaken in your more familiar environment. This will go a long way toward preparing your body to achieve the marathon goal time you’ve established.
Elite marathon runners who train at altitude use a similar approach. For example, the members of the Mammoth Track Club, who train at 9,000-11,000 feet, have a formula that calculates sea level equivalents to times they run at high altitude. Last year I watched one team member, Josh Cox, perform a long run at marathon effort at 9,000 feet. His marathon goal pace (for a sea-level event) was 5:00 per mile, but he ran the workout at 5:09 per mile—a slower pace but an equivalent effort. He came up a little short of his goal time in the marathon, but he did set a personal best and he got second place. All in all it was a great success that validated his high-altitude training approach.
Nevertheless, any given pace run at high altitude is never truly equivalent to a faster pace run at low altitude, nor is any given pace run on hilly trails in the heat truly equivalent to a faster pace run on a flat, paved surface in cool weather. Even if your heart rate and perceived effort levels are the same at different paces in different environments, the efforts are not really equivalent. When you’re running faster, your nervous system is working harder, your muscles are contracting more forcefully, your body is absorbing more impact, and so forth. Consequently, to fully prepare your body to run 10:30 miles in your coming marathon, you must run some 10:30 miles in training—and you also must log some miles on paved surfaces.
Therefore I suggest you complete a handful of longer runs at your target pace on that boring old bike path. Most of these runs should fall in the latter weeks of your training, when it’s time to put the finishing touches on your marathon-specific fitness.
Check out Matt’s latest book, RUN: The Mind-Body Method of Running by Feel.