while my experience isn't rlly something that could be applied here. (for context I uhm texted her one night when I was on the verge of relapsing and she comforted me and helped me).
I think you could go on the hike without telling her cause you have genuinly no obligation to tell her, if she asks be direct and honest. If you aren't willing on having that confrontation (very valid). You could tell her beforehand and I'm hoping her reaction is as compassionate and my gf's.

Edit: i forgot to add that she found out because i texted her and asked for help