Is there an effective way to pace for an outdoor ride without a power meter? Or a less expensive way other than power pedals or cranks?
Pay attention to your breathing rate in the different zones when riding with a power meter. Then take that knowledge to rides where you don’t have one.
And a heart rate monitor.
Before power meters we used heart rate for zones . It’s effective.
For long efforts, heart rate works well. But there’s a significant lag.
Otherwise - RPE. But this requires that you genuinely think about how you feel.
As others have said, HR can be decent for pacing. But there are pretty cheap pm options out there also. People are basically giving away old powertap hubs/wheels if your bike doesn’t have disc brakes. Single sided spindle-based sram is cheap if you have a crank that supports upgrade. But eBay/marketplace are where you can often find stuff used for not much more than a new HR strap.
For many years we just went by feel. I don’t recall ever hearing the term RPE back then. When HRMs became commonly available, I looked at it as a novelty, not as something I used while riding. After a ride I would look at it and say, “Oh, I averaged this BPM, or maxed out at that BPM.” When I finally got a PM, it was similar. I still rode by feel and only looked at the PM occasionally, but mainly after the ride. Quantitative methods like HR or power can provide a bit of an edge in some competitive situations, but even then how I feel can be more relevant and is how I ride.