Thanks for sharing your experience with elevated blood glucose. Good for you for monitoring your glucose level with a home meter. It is remarkable to me how many people on the pre-diabetes- diabetes spectrum do not monitor their blood glucose. Very important to avoid those spikes as much as possible.
I'll assume that your units are in mmol/L. Those readings seem high- especially if you are getting readings of 14 mmol/L. That is actually in the full-blown diabetes range.
Exercise can be tricky with pre-diabetes/diabetes. You definitely want to exercise regularly, as it increases your insulin sensitivity and in the big picture helps control blood glucose. In fact, regular exercise is one of the most important things that a person with insulin resistance can do to improve and control the situation. On the other hand, as you are seeing, for some individuals, intense exercise can cause blood glucose to rise for a period of time following the exercise. Your levels are probably doing fine during the exercise itself, as you’re burning up the blood glucose with your activity. However, you are correct that with intense exercise hormones are released which cause the liver to produce glucose and this can continue for some time after exercise cessation. For normal individuals with healthy insulin production, this is no problem, as their pancreatic beta cells kick out plenty of insulin to control the blood glucose being produced. But those on the diabetic spectrum generaly have significantly impaired ability to produce insulin, so there is very little to control the rise in glucose and, of course, the further someone is down the spectrum, the more impaired and the higher the glucose spike and the longer it will typically remain elevated. This can cause significant damage in all kinds of areas.
I think that you are wise to back off the level of intensity of your work outs. Something which you may want to experiment with is on the days that you train hard try adding a long cool down period at the end of your workout. Continue to bike, but just at a gentle constant pace. The idea is that the exercise should continue to burn off the glucose and keep it controlled, giving the glucose stimulating hormones time to burn off as well. If you bike for an hour intensely, maybe try an hour of cool down before stopping?
Ideally, maybe you can get your doctor to prescribe a continuous glucose monitor that you wear for a couple of weeks so that you can see what your glucose levels are doing during the entire work out and cool down period.
I think there is generally sound guidance here: Exercise Can Raise Blood Glucose (Blood Sugar) | ADA
Please keep us posted on how your modifications affect your levels.