I've hiked/run/jogged the loop several dozen times (I used the loop as my training grounds for my hike up Mauna Loa in the summer of '95) and have enjoyed each occasion. The trailhead begins at the top of Aiea Heights Drive at the Keaiwa Heiau State Recreation Area. You can either do the traditional clockwise route that begins next to a restroom building in the upper lot, or you can begin near the lower campground and hike the loop counterclockwise.
What's more, from the Aiea Loop, you can take trails leading to Kalauao Stream (I know of three routes one can take to the stream). In addition, there's a little-known trail that follows a ridge to basically bisect the loop.
If you do the Aiea Loop, be prepared for muddy sections, particularly on the upper end of the loop near Pu'u Uau (elevation 1,656 feet)--the halfway mark--and on the lower sections near the riverbed. As I mentioned earlier, the route is well-traveled and relatively easy to negotiate, so much so that when I do it, I run about 80% of the distance. And I'd run more if I were willing to risk jogging through mud-filled sections of the trail. Not one to imitate female mud wrestlers, I opt to tread gingerly through these bogs; I'd urge you to do the same.
Just past the upper halfway point, you'll have some nice view into upper Halawa Valley and the still-under-construction H-3 freeway. There's something surreal about being way up in the mountains and suddenly looking down at a brand spanking new tunnel and viaduct springing out of the base of a mountain in a pristine valley. In a few years when H-3 opens, I'll use it to drive to my job at Leeward Community College in Pearl City. On my daily commute, I'm sure I'll gaze mauka and smile knowingly, for I have been there.