I still think you're running too fast on your easy runs. You're probably running nearer your, like, half-marathon-marathon pace, and I get the impression that doing that for all your runs can definitely wear you out. I'm not an expert here at all, though.
you might be right. but just last week i ran 8.9 km at 4:41 and felt great -- it was an easy, relaxed run. today i died after 5 km at 4:51 average. then again splits were 4:41, 4:35, 4:34, 4:59, 5:01, 5:23, and then the last fraction even slower. you can tell what was downhill and what was uphill. perhaps what i need to do is just be even more patient with myself climbing steep hills. or stick to flatter routes.
also might just be continued recovery from the hill workout two days ago.
in any case, you're right that i should be careful to keep the easy runs easy and focus on mileage rather than speed, and then "run with power" (as adarq's training partner said) on the hard days.