Background: Mobile health apps show promise for delivering physical activity interventions, but uptake remains low due to usability barriers. Beta testing is essential to optimize user experience before full implementation. Objective: This study aimed to evaluate the usability, acceptability, and feasibility of the 50K4Life mobile app prototype for delivering a 2-week walking challenge to public school administrative employees. Methods: Following the Integrate, Design, Assess, and Share framework, we conducted a single-group beta test with 12 public school administrative employees in El Paso County, Texas. Participants used the 50K4Life app built on the Pathverse platform for a 2-week walking challenge. Data collection included acceptability surveys, satisfaction questionnaires, app use metrics, and qualitative debriefing sessions. Results: All 12 participants completed the walking challenge. Acceptability was high for app design (n=12, 91.7%), layout (n=9, 75%), and battery impact (n=10, 83.3% reported no issues). However, participants experienced difficulties with navigation (n=7, 58.3%), delays in updating step counts (n=8, 66.7%), and completion of assigned tasks (n=5, 41.7% could not locate all features). App use was high: 100% (n=12) accessed the leaderboard and walking challenge page, and 91.6% (n=11) synced step data and set step goals. Results from participants’ responses in the surveys and feedback from debrief interviews identified needs for improving user engagement features, synchronization, and visual appeal. Conclusions: The application of a participatory approach and Integrate, Design, Assess, and Share framework yielded valuable insights into the acceptability of the 50K4Life app reported by this study cohort and potential for enhanced features in <i>real-world</i> use contexts.
