Very nice day trip option.
The medium size fits my female collie (48lbs) very well. She's long, lean, and chesty which makes other packs fit her too tight in the front, too loose in the back, or have comically large saddlebags for her small frame when we try the size corresponding to her girth. Ruffwear packs are generously adjustable! Our male (68lb) collie wears a size large. We're looking forward to the cooler hiking weather in her new pack. :)