Ginni wrote in asking me how she should figure out “fast” rides and “slow” rides.”
The difference between fast and slow on the bike has to do with intensity, which is not necessarily linked directly with a particular setting or speed, but which has to do with how hard you are working on the bike.
Your heart rate and perceived exertion are indicators of this, but note that your HR parameters will be different on a bike than with running, because a difficult pace might not translate into as high a HR as you might think, based on how you feel when running. In other words, if you sweat and grunt on the bike at a level that feels like a certain HR you achieve while running, you might be surprised to find out that your actual HR is lower than expected. This is due to the fact that your body weight is supported by sitting down in the saddle. This doesn’t mean that you’re not working hard; just take that into account.
For this reason, I prefer to go by perceived exertion. So, on a scale of 1-10, with 10 being hardest, I consider an easy ride to be a 5-6, and a hard/fast ride to be an 8-9.5.
I don’t want to confuse things, but there is an additional, more objective measurement that could be used in cycling: monitoring wattage output. Simply put, HR and perceived effort measure what’s going on in your body, but wattage measures what your body is actually doing. It’s like noting the amount of weight a bodybuilder lifts, instead of how he’s feeling while working out. The higher the wattage produced in one hour, the higher the intensity. As your fitness develops in cycling, the wattage you produce goes up. Many cyclists train by wattage; I don’t, both because of occasional lack of access to data, but also because I’m satisfied by monitoring my perceived effort. I just mention this because you should know how wattage fits into our discussion.