My wife and I decide to hike to the Makapu'u Point Lighthouse when we were doing the "tourist" thing riding around Oahu. The sign at the entrance stated the walk was about 1.5 miles round trip... the mileage was off about a mile. I had a GPS unit which indicated about 2.6 miles from the entrance to the parking area.
Although the walkway is paved, it has a pretty decent grade all the way to the top. We were fortunate to have a stiff breeze in the 85+ degree heat. You cannot see the lighthouse unless you are at the top most observation area; however, on the way up, you do have one opportunity to see the lighthouse.
Please note there are no restrooms or places to get water along the route, so be prepared.