Ssc - Yes definitely. Less pressure. Re your 15 miles. My coach used to give me a 20 miler that was 10 easy then 10 MP, and would say if this was achieved it was a good indication that the MP was right for the day. However, if this was a struggle then MP should be adjusted. Of course this needed to be backed up with the endurance built from long runs.
Ssc - I think that sounds very promising. The 15 avg 7:30 - was that progressive, chunks or consistent? Personally I've never run with a pacer and never would. You want to back off if you feel you need to then ramp it up again when it feels good.
IM - in my opinion lots of race miles in those shorter runs doesn't compare with building endurance through the long runs, so for that reason I'd be careful of going off too fast. Better to start with some caution then push on in the second half/ last 6 miles?