Speed work revelation

So for my marathon training program usually once a week I have to do a "pace" run where I run at my planned marathon pace which is 6:04 per mile. About four weeks ago I started the first one with three miles which turned out to be extremely tough having not done any real speedwork in as long as I can remember. I think for Haney to Harrison last November I did some semi-intervals where I ran :05 x 5 at about a 6:30 pace. Here I was having to run 18 minutes straight at 6:04!! Well at the halfway mark where I turn around I literally had to stop for about a minute to catch my breath I felt so bad. That was running around the neighbourhood where it seems pretty flat when driving or riding the bike but there is actually quite a few elevation changes that are gradual - nothing but a few percent of grade but enough that your breathing changes and you can feel your exertion rising. This sucked so bad I wanted to find a dead flat road and maybe that would boost my confidence somewhat.
In the weeks in between I've been doing tempo runs which is helping the speed but mostly these are done on the treadmill because it's easier to control the changes in speed for short durations. I found that along 40th ave between King George and 184th is the flattest road around through farmland. I did two 6 mile pace runs and found them as hard or harder than the ones around the hood with the slight rises and I think I know why: it could be a mental thing that all you see in front of you is straight road and the finish miles in the distance. A lesser reason could be that there's usually a wind blowing some direction because it's very open along there. The biggest reason I now believe is that the elevation changes make different muscles work and I know the roads around the house so well. Having the knowledge of the roads makes it easier when I go up a rise and the pace slows, I know that I will be able to go down the other side and make up the time with less effort and it all evens out. All the corners as well keeps my mind more occupied than a straight flat road. Last Saturday was a 7 mile run that ended with a 6:00 per mile average and I actually felt pretty good about the whole ordeal. I remember years ago doing the longest pace runs, 10 miles, and finishing tired thinking how I was EVER going to do 26.2 miles at that pace. I chalked it up to not being fully rested or tapered because the marathon itself didn't get that hard until about 22 miles. Anyways, bottom line is that a rolling course seems to work better for me all around and besides, the marathon course ain't all that flat and the changes in elevation do me good.