Jan Wellford provided some perspective:
New York's highest summit is also one of the Adirondacks' most runnable peaks due to the easy grade of the trail. The Van Hoevenberg trail beginning at the Adirondack Loj is the fastest way up Marcy. It is very eroded, littered with rocks and roots and often wet, but at least it's not that steep. I'm sure Marcy has a long history of running, hiking and skiing records, but I am having a surprisingly hard time finding out the details. I'm hoping this post will attract some attention from those who might know.
I'll go ahead and claim the FKT for a Marcy ascent, 1:18:28 starting at the register on the "ranger trail" at the Heart Lake campground and ending at the summit (not the plaque just below the summit). I did this today (10/26/12), breaking my old time from May 2011 by over 5 minutes. The route follows the Van Hoevenberg trail for the entire time with the exception of the slightly shorter "ranger trail" start. It is about 7 miles and gains 3240' elevation.
My fastest round trip time is 2:31:16, set on 5/31/11 and run on the Van Hoevenberg Trail the entire way. The splits were 1:23:44 for the ascent and 1:07:32 for the descent.
It's also common to time oneself from Marcy Dam (the far side, closer to the summit). This allows a 2+ mile warmup and removes the question of which trail to start on (Van Hoevenberg, Truck Trail, or "ranger trail"). Now that the bridge over Marcy Dam is gone (as of fall 2011) the entire route is about 1min longer in total, so starting the clock on the other side removes this handicap. My time from the dam today was 59:32 (I ran easy to the dam, planning to stop and stretch and time myself from there, but made the decision to run through so that I could try to beat my time over the entire route).
I have heard of some other fast times on Marcy and wouldn't be surprised if it has been done faster than 1:18. Matt Diskin (former Summit Steward and collegiate runner) put up some fast times between 2004 and 2007: I've heard 1:23 from a colleague of his, but also recall hearing 1:11 or even "trying to break 1hr" (I have to assume that was from Marcy Dam, as sub 1hr for the entire trail would be world class). Lake Placid is also home to some Olympic nordic skiers as well as the Olympic Training Center, so I imagine it has been run by many very fast skiers. Edit: Matt Diskin found the FKT site and cleared this up in a post below: he ran 1:12 and was actually trying to break an hour from the Loj!
Speaking of skiing, the fastest known round trip time on skis is 2:37:40, set on 1/28/08 by myself (I also did 2:38 on 1/28/11, exactly three years later). My ascent time was 1:50:30 and descent was 47:10, both of which are probably records although Pat Munn has done the descent very quickly, I believe slightly slower but not much. My times were done on ski mountaineering gear before I had a Garmin, but I plan to go faster this winter and will upload a track then.
As for hiking, Pete Fish (former High Peaks ranger) has summited Marcy over 700 times. I'm sure there are many other crazy accomplishment on Marcy and hope to hear about them.
I met Peter Fish once on the summit, every bit as cool as you'd expect him to be.