I've had a boil for over 2 months inner inner thigh. grows, recedes a bit, grows more etc; never comes to a head. exercising daily. purplish in colour?

See a phyisician: Could be folliculitis, which could be constantly irritated with your daily exercise--i would start with topical antibiotic and a bandaid. Also, change your exercise habits for a bit to reduce the friction between the legs (for instance, biking might be better than running). If it does not get any better, see a physician--may need oral antibiotics, or it could be more than just a boil.