I'm too old to have used a heart rate or power metres, but always used a watch or computer. They don't make me try any harder, but knowing the time and distance, especially for the last mile to keep me motivated.
When I started, I used to study the course details and the distances they gave and calculate my target time for reaching those points. Then used to write them down on a piece of paper and cellotape it to my stem. In olden days, they also used to put up 5, 4, 3, 2 1 miles to go boards and it was a great incentive when you reached them. Now it's all done by the on board computer, which does the same function, but doesn't have the same motivation factor.
Now I only ride our evening 10 series and always on the same course, so I know every bump in the road and when to try a bit harder.