The only problem that you have with this technique is that you won't be able to "just know" without a computer. You can feel and "train" by RPE but you don't really know until you actually have the data.

You will have no luck performing specific targeted intervals with any sort of consistency without the numbers to look at.

I know that there use to be a time where there were no cycling computers Myosmith. But there also used to be a time where there were no computers at all. Both are available now and are valuable for their respective purposes.

You can buy a cycling computer for extremely cheap. If you really want to "train". And I mean training for races to improve your performance, to increase your threshold, then you need a plan that you, personally, can stick to. You would greatly benefit from the numbers and you need to know how to interpret them.

If your happy without a computer then keep riding without on and I'm sure you will be fine, but if you care to get faster I would highly recommend training with at least a speed and cadence device, preferably a heart rate monitor or even better; a power meter.

Ask any educated or trained coach about this question and you will get only one answer. And that is that you need numbers.