So, today I had an 8 mile run that wasn't supposed to be at a moderate effort, not going anaerobic, but not going easy. I decided to go 8 miles on flat at 6:15s at 5000', which I seemed to remember being about 6:07s at sea level, which is pretty close to the marathon pace I'd have to run to do a 2:40. It turns out, it corresponds to 6:02s on sea level, in fact about a 2:38 pace, according to the Jack Daniels Calculator that I am so fond of. I've figured that if I have a great day in December (assuming I continue to train well), I could probably hold an average of 169 or 170 for the duration of 26.2 miles. 6:15s at 5000', similar to 6:02s at sea level, got me an average HR of somewhere between 171 and 172 today, so I'm cardiovascularly right around being able to run a 2:41 or so right now if I can hold up the 169 or 170 the whole way. My legs probably can't hold up on roads well enough to run that right now, but I have 2 months and that's plenty of time to get my legs fully from trail form to road form.
My short distance speed and my ability to keep a lower heart rate at a fast pace is definitely getting better as well, which is a good sign. I did a 4.8 mile tempo on roads the other day (was supposed to be 5, I guess I'm cheating... haha...). The run had very serious hills the first 2 being straight uphill with 600' gain in 14:30, and from 2 to 4.8 rolling downhill (net 600' downhill, but about 250' up, 850' down) in 14:50. Obviously, despite the rolling nature, this is going to be faster than flat, but the 5000' elevation still costs me time. The last 2.8 corresponds to 15:20 on flat at sea level, or just under a 17:00 5k. This doesn't sound fast, but my heart rate averaged only about 177 and never crossed 180 except for the last 1/4 of a mile or so when I picked up my pace to 4:40/mile on the heavily downhill finish. In an actual race, I'd average WAY higher than 177 (actually averaged 191 in a 10k), so the fact that 177 puts me only 25 seconds off pace for a 5k PR during the last 2.8 miles of a run means that I could destroy my marathon PR on a good course right now.
Back to thinking about the marathon... I think with some good training focused on roads, I should be on track to be able to get right around that 2:40 point by December, though I'm sure it will take everything I have to race a marathon in 2:40 this year. I think my half marathon I'm running 2 weeks out from the race should give me a good idea, but I really think it should be possible at this point.