Half Dome is one of the great peaks in Yosemite. The hike to the top of Half Dome is 17 miles, round trip. That's not too much further than a half marathon, but the last 500 yards to the top involves pulling yourself up along a set of cables.
Obviously, the Mister and I have decided that this is the sort of thing that we should undertake. Our tentative climb date is June 8th, assuming that we get permits.
I've been reading some very helpful websites that have made me feel that this is actually possible for a mere mortal. Kenton Lee's review and Mr Half Dome's blog have all been great resources so far.
My "training plan" is mostly running, some elliptical, weights, and lots of long, long walks and hikes. Living in a place that's a) fairly flat and b) fairly freezing doesn't allow for too much in terms of similar conditions!
For some pictures of the actual route, check out the Yosemite Hike website (pictures are via the Photos link).