Together with Watts, since HR alone offers no control over technique and style. Ergdata connected to a PM5 shows us length and handle force, as well as Power and rating, so we get a stroke by stroke analysis of causes and effects.How do YOU use your heart rate data?
HIIT if long enough (3 x 10 x 10 once a week?) might be able to compensate fat loss with muscle growth, if that's your aim.
LIkely not your case now, but in due time you'll notice that metabolic rate and power output drop with age. There's nothing wrong with fruit and veg. In any case, having given up racing, sooner or later we are exercising enough to maintain fitness; in that steady-state we take account of the first law.