I think they don't want us to have more information than we can handle! If you're exercising a lot now, though, perhaps you could talk to your leader. During one of my WW rejoins
I remember a very active newcomer who was working out 2 hours a day at a gym, and the leader told her to eat more, and how to figure that out.
How many points you get for what activity depends on your weight, so I don't think there's an easy 20 minutes = X. For me, 30 minutes of brisk walking is 4 points, while 30 minutes of leisurely walking is 2.
You can use up to 4 activity points a day to spend (eat) or bank.
HTH.