In reply to JH74:
There was an article in New Scientist about the optimal way to increase fitness and it said HIIT sessions. I quote the concluding paragraphs
"High-intensity interval training (HIIT) involves exercising at near maximum effort in repeated bursts of up to a minute, with short rests in between. This regime was devised in the 1990s by Izumi Tabata and his colleagues at the National Institute of Fitness and Sports in Kanoya, Japan. They showed that a 4-minute workout, with repeated cycles of 20 seconds of intense work then 10 seconds of rest, done five days a week for six weeks resulted in greater aerobic improvement than moderate, hour-long workouts done over the same time frame.
Since then, many trials have confirmed the power of HIIT. “If you want a quick increase [in fitness] over four to six weeks, purely intervals would do the job,” says Lane. The key is working as hard as you can during the intense intervals. Just six HIIT sessions over two weeks significantly improves VO2 max and endurance capacity, but, remarkably, a 2021 study found similar improvements could be achieved if these HIIT sessions were squeezed into a five-day period.
Adam Sharples at the Norwegian School of Sport Sciences in Oslo says beginners should start with two to three HIIT sessions per week. You can intersperse these with longer-duration endurance activities, such as jogging or swimming, to further boost results, he says. Once you notice that your strength has plateaued with HIIT, you can add two or three full-body strength-training sessions, says Lane.
However, the best exercise is ultimately the one you will be able to stick with, not necessarily the one that leads to the quickest improvements. “We sometimes get in the weeds with what is the perfect plan, but I think for most people, we just need to get out there and get moving,” says Gray.
The trick is to continuously challenge yourself. If you do, you may be surprised to discover your own peak performance."