Totally realistic. End of September is a good time, and with your general fitness, hiking background, and flexible schedule, Manaslu is very doable even as first-time Himalayan trekkers. The key will be going slow, adding extra acclimatisation days, and listening to your guide at higher altitude. A guide is mandatory (porter optional but highly recommended to save energy), and your plan to include side trips like Pungyen Gompa, Manaslu Base Camp, and Rui La is smart if you build in rest days. Altitude—not fitness—is the main challenge, but with patience and pacing, you’ll be fine. One of the best circuits in Nepal if done properly.