Confused about my period ending, can I fast tomorrow?

Assalamu alaikum, I’m trying to make up a missed Ramadan fast and need advice. After work yesterday my period looked totally finished, so I took ghusl and started praying again. Problem is, occasionally a tiny bit of blood shows up the next day even when I thought it was over, and I end up doing ghusl again just in case. I'm training for a marathon right now, and tomorrow is a rest day-perfect time to fast. Obviously Allah comes first, but it would be way easier. What worries me is if I do ghusl before Fajr, begin my fast, then my period returns during the day, I’d have to break it. And I’d be at work without bringing food, with no drinking water there. I'd need to go out and buy something to eat and drink to break the fast. Would you fast if you were me, or wait an extra day to be certain? Jazakallah khair!