This week I did four consecutive runs of an 8.24 mile course. It was just bad luck that we are having a bit of a heat-wave now. It is only in the mid-80's, but very humid and I do run in the early afternoon.
My times were not great: 8:30, 8:20, 8:21 and 8:12 were the average paces. Normally I am unhappy if things are above 8:00 and really, below 7:30 is what I feel like should be happening.
The heat alone should account for some of the slow pace, but I think there is another cause as well. This route has a section of bike path that is made of packed gravel. I know how long each of the segments are and so I know that the pace before and after the bike path are solid 8-minute miles. At an 8-minute pace, it takes 20 minutes to get to the path, 16 minutes on it and then 30 minutes back to the starting point. All the losses are due to the inefficiency of running on gravel. The heat is somewhat less on the path since it is narrow and wooded, thus shady. There is less of a breeze in the woods though.