Since this is a 50+ post, two HIIT sessions a week may be too much with another 3-4 rides a week. Try it and see.

Last year I felt stalled after two years back on the bike, riding hybrids at a mostly casual pace. I was faster on flats and could ride longer distances, but was still thwarted by hills. So last June I bought a good used road bike and started HIIT, following some of the variations suggested on GCN videos. My main goal was to improve my stamina and recovery time on climbs.

I made rapid gains and was finally able to finish difficult hills that used to force me to stop at the top to use my inhaler and catch my breath for a few minutes. And I was a little faster on lesser hilly routes. I was able to ride 20-30 mile roller coaster routes without stopping to rest.

But after 2-3 HIIT sessions a week I finally hit a wall in August-September and was suddenly exhausted. I was over-training, not getting enough protein, not getting enough rest.

I cut back to one HIIT session a week, then longer rides at a sustainable effort on everything else. By November my speed was back, and I felt like I hadn't really lost anything. My speed was better on some longish personal segments of 2-10 miles, and I even cracked the top ten on a few Strava segments although that wasn't a priority. It just happened.

I added more protein supplements after reviewing and tracking my diet -- I was way under on protein and total calories for the effort expended, which surprised me. Turns out just eating when I felt hungry wasn't good enough.

After getting the flu in late December-early January, I stopped HIIT completely. Just one casual ride a week was all I could handle. I finally feel back to normal now, almost two months later. So I'll start one HIIT session a week next month. Not gonna rush it because I had so many relapses with flu symptoms this winter.

Incidentally, you may find supervised HIIT training on an indoor bike to be different from outdoor, especially solo. With solo outdoor training I usually stopped just short of maximum possible effort because it's dangerous to put out a true 100% effort. If you push to maximum possible effort outdoors there's a risk of falling while being momentarily dizzy, nauseous and disoriented.

According to some trainers and participants, a true HIIT effort will hurt and make you feel momentarily sick. So even if you already have good baseline fitness, and your heart, etc., can handle HIIT (be careful if you have high blood pressure, history of strokes, etc.), it's risky on a solo bike ride outdoors. If you watch the GCN videos, the presenters (usually Matt, Simon and Daniel) will work together in outdoor sessions and two of the three will hang around the end of the maximum effort zone to assist the one doing the maximum effort to help them with balance.

I almost always ride solo outdoors and don't have an indoor trainer or go to the gym. So if I plan on a 10 second or 30 second maximum effort sprint or climb, and feel like I'm about to redline sooner, I'll ease up. I'm still getting the benefit of the harder training, compared with my former practice of emphasizing distance over effort.

And I'll occasionally do HIIT exercises at home with shorter rest periods, because the risk of falling is much lower. But on the bike I'll rest for a full minute or longer between efforts. I'm not training for competition, just to get fitter to enjoy my rides, so there's no point in risking passing out and falling. However, I do wish I'd known about HIIT when I was younger and an amateur boxer. It would have improved my stamina for the three 3-minute rounds.