The above data are using my new Marathon Predictor calculator based on Greg Maclin's algorithm. The Marathon Difficulty Factor (MDF) is based on the elevation profile only, not turns (which matter) and altitude (which matters, but not going to affect the New England marathons).
Don't like my 3 hour example? Your expected pace for any of these marathons is simply pace*MDF where pace is your pace on a flat course. I've worked out an example using a 3 hour flat marathon above. If you've run one of these marathons and want to know what the expected pace on another is, your unknown pace is pace(known)*MDF(unknown)*MDF(known). Very simple!
As I showed in my previous post, expected pace and time are a function of the hills and this is where my calculator differs from Maclin's. First, I'm not sure where he got his elevation profiles but at least some I think were obtained with a barometric altimeter on his Polar Watch. I would think this would nail it but he has some odd stats that kinda hit you across the face when you stare at his chart. For example, his total gain for Manchester City is only 100 feet more than Baystate. Based on everything that I've read (mostly blogs but also estimates from the various online mapping sties) this must be far from accurate. Also note that Maclin has the net elevation gain/loss for the Boston Marathon as -378 feet but a good look at the elevation profile provided by the BAA shows this is closer to 450 feet. Marathonguide confirms this. Given these are the only three marathons that I've looked at closely (Baystate, Boston, Manchester City), I don't have as much confidence in Maclin's elevation profiles as I have in mine. One other difference between our algorithms is that I use a 0.01 mile window to compute grade and pace not a 0.1 mile window. Since I smooth my elevation profile, I'm not worried too much about overestimating the MDF and I'd rather not miss important peaks and troughs that can occur well away from the 0.1 mile marks.
Thanks to Jim's suggestion, my elevation data come from the USGS NED database based on gps positional data during marathon races for runners running about a 7 min/mile pace. That is, I substituted elevation data from NED for the gps/satellite data. I found the marathon data from the old motionbased.com site. The data are then smoothed using a cubic spline and a smoothing parameter of 2.51E-04. This smoothing parameter was chosen based on a very detailed comparison of each of the hills on the Maine Marathon smoothed elevation profile and the Google USGS topo map (since I'm familiar with this marathon, this proved fairly painless). I used the same smoothing parameter for the other marathons.
I've got a system now that I can very quickly compute these for any course that I have a gpx file so I'll add some more starting with those that are most relevant to New England.