Maybe I misunderstand it but if 12:40-13:20 is your easy pace, then 174 is never your max HR. Your max HR would be with a fast pace. I think if you would run 3x 3 minutes as fast as you can (2 minutes slow running between) you will have a higher HR than 174. And I don't think that nearly walking for keeping the HR down is something that increases your pace. It's better to use pace. A slow pace is when you can talk without problems, one breath on 4 steps. A medium pace is a bit harder to talk, one breath on 3 steps. Fast pace is hard to talk, one breath on 2 or 1 steps. If you do that several times you can see which pace range corresponds to easy, medium or fast.
HR can drift due to a number of factors including heat, hydration status, fatigue etc. Pace will/should stay reasonably constant if you run on the flat. Your easy running pace sounds perfectly reasonable so I'd suggest you stick with that. But, that pace will only apply on the flat for that particular heart rate. If you do anything other than flat, you will have to adjust your pace accordingly. When you are running at pace on the flat, think about how you feel - your perceived effort. Use that perceived effort too adjust your pace when you are on anything other than flat.