Recreational / Hobbyist UAS pilot reported flying in a restricted military alert area without authorization.

Narrative

On Day 0; I operated a Small UAS for approximately 15-20 minutes along the coastline of the trail. My flight path was primarily over the coastline and occasionally over the water. While my maximum altitude reached approximately 80m (262 ft) AGL; I spent the majority of the flight at a much lower altitude; well below the 400 ft recreational limit. I was accompanied by three friends who acted as informal Visual Observers. During the flight; they spotted a manned aircraft flying at a much higher altitude. Although the aircraft was well clear of my position and way above me; I proactively lowered the UAS even further as a safety precaution until the aircraft passed.After landing; I realized I had inadvertently entered the NAS Whidbey Class C / Alert Area 680 without prior authorization. While I typically verify airspace restrictions on a planning app before every flight; this instance was a complete oversight due to being distracted by the scenic beauty of the landscape. No safety hazards; property damage; or injuries occurred.Corrective Action and Prevention: To prevent recurrence; I have updated my physical pre-flight checklist to include a mandatory 'Airspace Go/No-Go' step. I will now use a dual-verification method by checking two independent airspace awareness applications (e.g.; LAANC and Drone Buddy). Furthermore; I have adjusted my drone's flight software settings to permanently limit the maximum altitude to 60m (197 ft) to provide an automatic safety buffer below the legal limit. I have also briefed my frequent flying companions on how to assist with identifying potential 'No Drone Zones' using these apps before we launch.