Friendlypain,
Pardon my ignorance, but why do you assume it is a hip flexor problem, rather than an adductor strain?
Assuming it is a hip flexor problem, your advice to jog out a sprint is impractical - the defender needs to track the player he is marking, which will involve multiple short, sharp stops and starts.
Megmoo, I would suggest that whatever you pulled you returned to sport too soon, without having done any appropriate rehabilitation. Bite the bullet and get some advice/treatment from a good sports physio in your area, it will be money well spent in the long run - you don't want to get in to a cycle of groin injuries, which can severely hamper your ability to enjoy your football.