Those are some great questions your raise. I've been running consistently for about 1 and a half years. I was completely under-trained for my first marathon. I ran 8:10-8:15 splits, very easily for 20 miles, then I struggled severely the last 10k. I think it came down to time-on-feet. My legs were shot.
A lot of the problem with effort is the heat/humidity here. The effort to run a 8:30 pace for 20 miles is exponentially more difficult when the temps are in the upper 80s vs. race day conditions in the 40s.
As you suggested, I plan on running 15 miles this weekend around 7:45, 7:30, after a warm up, to see how I feel. I think this is the best way to find a realistic pace. However, I feel like I could run a lot faster/longer even in temps in the 60s vs 80s.
Ironically, my Boston qualifier is 3:10. With my lack of marathon experience I don't think I can run 7:15's for 26 miles, realistically.
I was leaning toward running with a 3:20 pace group- or should I try to run faster?