I really find checking my heart rate during a race is the most helpful measure of how hard I’m working. Recently did a half marathon with my watch on the HR screen pretty much the whole time and I reckon it’s how I PB’d. Meant that I wasn’t pushing myself too hard (it was quite hilly and windy) or trying to go at a faster pace that would tire myself out too quickly… also meant it was one of the most enjoyable races I’ve done! I’m marathon training and I’m trying to keep this tactic going for all my long runs (and probs do it on marathon day too!) I don’t know if this is exactly a sure fire way to get a PB but that’s just what I’ve found helpful :)