I would caution against making direct comparisons between heart rates while running vs cycling. A runner trained in both disciplines will almost invariably have a higher heart rate while running. It is a weight bearing exercise and presents different demands on the aerobic system. When reviewing your heart rate on the bike you should be focusing more on elements like cardiac drift and recoverability than raw HR values. What’s happening to your HR at hour 1, 2, 3 etc… is it climbing? This is where endurance work is pivotal. Building those fatigue resistant slow twitch muscle fibers will allow you to work in your aerobic zone longer without drawing on Type 2 (less fatigue resistant) muscle fibers. It’s when you start to tap into those fibers that you start to break down. That’s the limiter, not heart rate.
Your heart rate is a measurement of strain, not stress. It could be a leading indicator of fatigue and there is much to be gleaned from it, but to look at it as a factor that could “limit” your performance on the bike is an approach I would recommend against. If you have weaknesses in your profile at the higher zones, that would be a driver to incorporate additional high intensity in your plan. However, trading off endurance for intensity based on comparing your heart rate between disciplines will likely lead you down a less productive path.
Hope this helps.