This study aimed to compare the perceptions and opinions of sedentary adults regarding two physical exercise programs, one in which an instructor applied motivational strategies based on self-determination theory with the aim of encouraging healthy habits, and another in which no guidance of this kind was offered. We examined the influence that the instructor’s intervention had on the perceived benefits of the program, the ability to overcome barriers to exercise, users’ views regarding the instructor’s role, and social relatedness in the group. Participants were 56 sedentary adults (8 men and 48 women) aged between 18 and 60 years (M = 36.02; SD = 10.15) who enrolled in a six-month program of moderate exercise, comprising three 50-minute sessions per week. Two groups were formed: a guided group in which a fitness instructor applied motivational strategies with the aim of promoting self-determination, and a non-guided group that received no such input alongside their exercise routine. A mixed methods approach was used to analyze data from questionnaires, interviews, and the diaries that participants kept throughout the program. The results showed that participants in the guided group reported: (a) more positive views regarding the benefits of the program, (b) greater satisfaction of basic psychological needs, (c) fewer barriers to regular exercise, and (d) the experience of positive relationships in the sessions.We gratefully acknowledge the support of INEFC (National Institute of Physical Education of Catalonia) and two Spanish government projects (Ministerio de Economía y Competitividad): a) La actividad física y el deporte como potenciadores del estilo de vida saludable: Evaluación del comportamiento deportivo desde metodologías no intrusivas [Grant number DEP2015-66069-P]; b) Avances metodológicos y tecnológicos en el estudio observacional del comportamiento deportivo [PSI2015-71947-REDP]; and the support of the Generalitat de Catalunya Research Group, GRUP DE RECERCA E INNOVACIÓ EN DISSENYS (GRID). Tecnología i aplicació multimedia i digital als dissenys observacionals [Grant number 2014 SGR 971]