I've now run 12 official marathons (not counting the 2 ultras I've done in the last few years, and one that I ran/hiked in the mid-90s). Finishing times can't be reliably compared to one another since there are so many variables, including: how hilly the course, weather, temperature, nutrition, fitness level, terrain/course surface, number of tight turns, and quality of training. But it's really hard to not compare anyway.
Reflecting on the four that took me longer than last weekend's 4:01:53 trail marathon (which clearly OUGHT to take longer than a road one for the simple reason that it's trail not road), here are my reasons/excuses for why they were slower:
1997: Mayor's Midnight Sun Marathon, Anchorage, AK (4:19:00)
During the training for this one, which took place on the June solstice, I pulled a hamstring around mile 18 of a 20-miler about 3 weeks beforehand, and didn't run a step during the 21 days in between. Not too surprising, since the race came only 3 months after my fastest marathon, at the Shamrock Sportsfest Marathon in Virginia Beach (3:22:00). I cramped pretty bad somewhere around mile 19 and basically limped to the finish.
2006: Key Bank Vermont City Marathon, Burlington, VT (4:03:29)
I blame this one on the heat. After a record cold spring where I don't think I got to run in shorts and a t-shirt more than once, this Memorial Day race took place on a sweltering day where the temperatures climbed well into the 80s. I started well but suffered greatly in the later miles. Basically, I melted.
2008: Around the Lake Marathon, Wakefield, MA (4:12:38)
This one was a "novelty" race that I didn't really take seriously anyway. For one thing, it takes place at night, in the middle of the summer. For another thing, it was really just an extra-long long training run that I was using as build up for the Clarence Demar Marathon 8 weeks later. (photo from a later year here)
2008: Clarence Demar Marathon, Keene, NH (4:10:17)
I actually don't know why this one was so slow. Maybe it was the extreme humidity. Or maybe I was just taking it super easy to make sure I finished; I had to pull out of this race at mile 19 in 2007 due to a stress fracture in my foot, and I really wanted to complete it. I needed to finish it, for lots of reasons. (photos here)
Still, Stone Cat featured rugged trail terrain and endless twists, turns, and short, steep hills. Plus I fell/crashed hard, twice. And there were no mile markers. And I went into it with a seriously compromised Achilles. It should have been my slowest marathon. Which makes me feel extra good that it wasn't.
The other seven, from first to last:
1995: Marine Corps Marathon, Washington DC (3:44:00)
1997: Shamrock Sportsfest Marathon, Virginia Beach, VA (3:22:00) PR
2004: Mount Deserst Island Marathon, Bar Harbor, ME (3:44:15)
2005: Tucson Marathon, Tucson, AZ (3:42:59)
2006: California International Marathon, Sacramento, CA (3:33:22)
2010: Marine Corps Marathon, Washington, DC (3:36:50)
2012: Jacksonville Marathon, Jacksonville, FL (3:36:11)