I also am currently thinking MT may have been taken by someone she did not know while out walking or running but that it happened the night of the 18. My reasons for not thinking it happened early on the 19 are that I don't think she would have gone to bed wearing her sweaty running clothes from the night before, nor put those same clothes back on in the morning. As far as we know, those clothes haven't been found, so she would have been wearing them. If it was raining (and possibly chilly too), I don't know that she would have set out in just a sports bra.
Also if it was raining, would she have worn her phone in the armband that we believe is missing too? I don't typically run in the rain but if I was setting out in the rain, I would personally make sure my phone was tucked away somewhere it would definitely not get wet at all. Just my thoughts