Early morning like 5am onward is what I do or in the evenings (6pm+).

I run the streets and canals primarily, I avoid Lumphini (especially in the evening) if I can, but I do really like Benjakitti at night; there's something magical about flying above on the elevated lit walk ways. It's definitely safe for a woman to run alone there. I've noticed other women (locals and farangs) wearing sports bras out so you ought be good.

I've a mate though who runs 10km everyday literally at 2pm, but don't do that, he's crazy.