Have you tried doing something else at the same time?
I struggle with this too. My longest runs of success have been when I combine the exercise with other things like walking on the treadmill while watching TV. Another one that tends to work for me is listening to an audiobook while I’m biking. Keeping my mind busy seems to help.
The only other tactic I can suggest is a bit more extreme. When I have been really desperate to ramp up exercise, I have made decisions, like selling my car, to force my future self to walk. It doesn’t necessarily have to be so drastic but to make it work you have to commit to something now that forces your future self to act. This is a technique I use when I really want to overcome ADHD related barriers like low motivation, boredom, and￼ impulse control.