If you're just riding for 'fitness', forget about the HRM. If you still want to track your miles because it's fun, just get a cheap bike computer, non cadence needed. Just ride hard when you feel like doing it, and enjoy the rest. The metrics are really only important when you want to know exactly how much you're improving and what kind of stimuli you need to keep improving, which is mainly for racers. It's overkill for fitness cyclists unless you specifically enjoy this sort of stuff.