Increase power output until your heart rate stops increasing. When power goes up and heart rate doesn't you've found your personal maximum. This is around where things happen like tunnel vision and vomiting. In formal settings they use a ramp test where you increase power output at 25W per minute.

That said the number isn't relevant. Efforts you can sustain for more than a very short time relate to your lactate threshold which is where you're making lactic acid faster than your body can process it and buildup starts increasing with the fourth power of effort - try 10% too hard, and it'll build up 46% faster. Where that threshold is as a fraction of VO2Max at your maximum heart rate varies with training and between individuals so even an accurate maximum heart rate is not a useful starting point.

Your lactate heart rate is about what you get averaging the last 20 minutes of a 30 minute all-out effort. Joel Friel has decent zone definitions built around that.

Finding 10+ miles of uninterrupted road and staying motivated through the discomfort
may pose an issue in which case you can use an alternate zone system from Chris Carmichael built around a pair of all-out eight minute efforts that are easier to accommodate logistically and psychologically.

With power measurement you use use critical power with CP60 (as in the power you can produce for 60 minutes) close to lactate threshold and something which can be estimated from somewhat arbitrary longer and shorter intervals.