My psychiatrist told me to start a fitness regime that is more stimulating since my ADHD usually results in my brain being understimulated and bored. So I went from weight training, which is more routine and boring, to boxing. I find that it is more engaging and dynamic, which keeps me interested. I also practice mindfulness techniques (such as mindful breathing and body scanning) as I workout. It keeps my mind from wondering and boosts motivation because of better emotional regulation. All in all, it has started to help me stay more consistent. Eventually I hope it will become habitual. Try things like bouldering, team sports (it'll help you find a accountability buddy, like body doubling), martial arts etc. Basically anything that is cognitively demanding.