I'm a 21 year old male training for a marathon in October. I hope to run a 3:10 or better in order to qualify for Boston. Obviously I want to be in the 6:30 to 7:00 mile range for this sort of thing, and everything I've heard says you should do an easy run at 1:00 to 1:30 slower than your goal race pace. Yesterday I did 15 at 8:20 average pace. I felt fine aerobically (not out of breath, very under control, very consistent pace), but my average heart rate was 175 (my max is 208ish). In terms of % of max heart rate, this does NOT seem like an easy run. However, if I was able to sustain this for 90 minutes + without discomfort, I have to be below my lactate threshold, right? Does the high HR just mean I'm out of shape (I've been on a few week hiatus from training, and previously I was only doing around 30 miles per week).
Any tips would be great. Right now I'm working on building up a solid aerobic base, so I'm hoping to figure out the best pace for running these sorts of things.