70-85 seems a bit high (to me) for someone in their early forties. I'm the same age, little exorcising and a bpm of high 40's - low 60's resting. Of course "little exorcise" might make the difference.
You may be well-conditioned physically and your heart is strong enough to pump an adequate supply of blood with fewer heart beats. If you have symptoms such as dizziness or feeling faint you should contact your doctor.