B737 pilot reports descending early on an ILS approach in VMC possibly due to fatigue after a very challenging day of flying.

Narrative

[We received] extended vectors 20 mile final after a long day. Weather was scattered and we were VMC. [We were] cleared 3;000 to intercept final 'cleared ILS'. I noted 3;000 until the first intersection; and then non ILS habit pattern set 2;300 for the second intersection thinking we were already past the first intersection. The autothrottles were not maintaining commanded speed selected and I was over riding to maintain speed and troubleshooting reason for throttle problem. I started down with runway in sight and cleared to land. Once I realized we had not reached the first intersection yet I disconnected autopilot and corrected altitude and continued to configure for 30 flap and uneventful landing. Be aware of fatigue onset. We had a very challenging day at SNA. BC LOC 1L; wind shear. After engine start at SNA ready for bleed off balanced field dept on 1L Tower switched runway; new clearance; TPS; FMS; etc; etc; etc. Even a generic ILS can bite you if you let your guard down. I will ensure better discussion of step down or altitude changes before initiating. We could have also just called the field and accepted a visual approach clearance. I will ensure not to confuse Non ILS procedures with ILS.