if you are taking mcmillan's prediction as your marathon pace then be warned- it's only. Well trained marathoners who seem to be able to do that. Many find their marathon a good 10-15 mins north of that calculator and pace accordingly.
Your biomechanics do change as you slid down to aerobic base training pace. If you also find it horrible to run that slow it may be that you are recruiting your fast twitch/anerobic capacity but your aerobic base may be underdeveloped. Well worth putting some focus on it if so.
Suggest you check the HADD training and / or P&d threads in particular.
Usually that slow pace is meant as the optimal way to stimulate your body to capillaries muscles and run aerobically. if it feels wierd running that slow It can be a sign that that system is comparatively underdeveloped.
7min miles is almost a 3hr marathon. that's quite advanced running by most peoples standards. You must have heard this stuff about slow running and differentiating your paces before.
I use treadmills on a 1% incline due to time constraints and road run at weekends. It's been fine to get going but I'm realising now I really need to shift the balance towards road running. my running is at a point where I appreciate the difference between the cushioned treadmill and the hard uneven road more.
Nevertheless, at no point was there a 10 min gap between by treadmill 10k and my road 10k. Not at the same level of effort. Something is wrong there. is your PB old and out of date? are you sure about the paces and distances you run to on the road?