The Waimalu Ditch is described in Stuart Ball's hiking book. It travels mostly along an old ditch into Waimalu Valley, and stops at the Ditch intake. It is less than 2 hours one way.
From there one can continue. In a half hour, after several stream crossings, one reaches the tongue of a ridge that comes down from the Koolau summit and bisects the Waimalu Valley. We call it the Waimalu Middle Ridge. (There is another "middle ridge" inside Moanalua Valley that is quite similar; that's described by Ball under "Keahiakahoe").
Ball and friends opened up the Waimalu Middle Ridge in 1991 or so. We did it in the summer of 1995.
From the bottom we start with a very stiff climb. Ball told us that this used to be completely trackless, with uluhe taller than a man. After 10-15 minutes it flattens out at several ironwood trees. Then the trail goes up and down gently for half an hour, and then another stiff climb begins, which lasts about 45 minutes. At the end there is a small open space where another trail goes down to the right. This is now 1.5 hours since the bottom.
Choose to go straight to go for the summit. The trail just goes gently up and down, mostly up, in the typical Koolau trail fashion. It is not particularly difficult, except that the trail is not very well cleared. At maybe 3 hours from the bottom there is a fairly narrow spot that one has to crawl along. At maybe 3.5 hours, the ridge trail ascends steeply up a narrow ridge, with a steep landslide dropoff to the right.
The first time, I stopped before this narrow ascent. A few weeks later, I went with a friend, with preparation to camp out. Unfortunately my friend was out of shape and very slow, and eventually he volunteered to stop and stay at the ironwoods near the bottom, but it was past noon. It was almost 4 p.m. when I got to the narrow ascent again this second time.
The ascent is not particularly dangerous, as the left side is gentle, and there are lots of bushes to grab onto. After this ascent, the ridge flattens out and ascends again up a narrow grassy ridge. At the top, the ridge flattens out and becomes a plateau topped with a beautiful short grassy lawn. It was 4 p.m. already, and the winds were howling, and I had trouble even standing up. The plateau ascends gently, presumably to a viewpoint of the other side, but I figured I needed 3 hours (7 p.m.) to get back to the camping spot, and I reluctantly stopped. Everybody who has been up to the very top says it is mystically beautiful.
I plan to return sometime this summer.