I've been doing a bit of endurance training this spring. Not only in terms of trail running, but also for climbing. Over the winter, bouldering dominates with short, powerful moves. However, now I'm trying to get back into alpine shape for the upcoming season, and to do that I have been doing laps at the Gutter.
Located up on the Second Flatiron, the Gutter is an excellent place to build up strength and endurance. At ~80 feet long, this forearm pumping problem will get you breathing hard and searching for your chalk bag in no time. With two cruxes, some real good rests, and a steep, sustained overhang the gutter is one of my favorite places in the spring.
The last time we were there I tried to get a couple pictures, but the place is hard to photograph. As you can tell from the only one that "kinda" turned out, the gutter is a slot that you can either climb "in" or on the "lip" for a range of 12b to 13a roughly.
Shady in the afternoon, I'll probably hit the place up a couple more times before it gets to hot and the higher stuff opens up.