Interesting thread, and full of good ideas.
I just wanted to second what was mentioned above about the HADD thread. Very interesting. Google "Hadd document" and you'll find a nice, concise write up on it.
The other thing I wanted to mention was about heart rate. One other poster suggested keeping your easy days easy. I've been experimenting with this for a few months and have found it to be extremely interesting. It is only an experiment of 1, and the results are not really in as my goal race (a 50k) has not happened yet.
I felt like I was reading everywhere (on more than just running forums) about HR training, and mostly the Maffetone method. Maf gets a lot of crap for how rigid and slow it feels, but I figured I could be patient and give it a go. At 35 years old my Maf HR was 145 beats per minute. So in theory all my training runs when I'm in the base phase should be below that.
I had been running for a few months, coming back after a long lay off, and I was consciously keeping everything easy, cruising around an 8 minute mile. When I got my HR I couldn't believe how slow 145 was. I had to walk a few times. OK, more than a few times. I even started giving myself a +5 buffer just so I could keep going.
After 3 weeks I was pissed at Maf and started looking at other HR theories (this is when I found Hadd) like Heart Rate Reserve and coaches like Daniels, Friel, etc. I put together a list of aerobic range HRs I should be in, and they were all between 142 and 151. So Maf was basically right in the middle and I went back to that and slogged through some miles in the high 9 minute range.
But then after 6 weeks things started feeling noticeably different. I was doing 90+ minute runs feeling awesome. Really, really awesome. They were not taking anything out of me. Most of my runs were on the trail so I had given up on pace, but when I revisited the bike path with mile markers I saw I was doing about 8:20 minute miles and staying in the mid 140s. It felt good.
Another 4 weeks went by and I started noticing my HR was more often in the 130s than the 140s. I went back to the bike path and I was holding 137 to 139 and running 7:50s. No one in their lets run mind would say that is fast, but it was the fastest, and easiest, run I'd ever done. And things are still getting faster.
So consider checking something like this out. From what you've said about slogging through your miles, not really finding an hour run easy, and guessing that your easy days are not easy, you might find spending a little time running by HR can help you build your aerobic engine and build your miles at the same time.