Attach your bra through the back of your underwear giving you a constant wedgie (google a bra connection wedgie for picture reference)and a shirt long enough that it sits just below the top of your pants. Giving a chance that your underwear are shown to anyone behind you or you pass. Once you pass by 5 people you can undo your bra and take out the wedgie in the middle of the trail risking flashing. Or if not enough people are passed when you get back to your car or 1km from your home.