[arethuza]

"what is one unit of exercise"

Based on my own recent experience I would suggest the "dog".

[taneq]

Addendum: if not a dog person, may I suggest ‘motorcycle’ as an alternative?

[arethuza]

This chap certainly recommends it for his worries: https://www.youtube.com/watch?v=S83zwxkNio8

Edit: A motorcycle that is, unless someone has created 200mph dogs...