Hello, Boulder909 – Of course, you can do whatever you choose on a fast day, but what’s the point of expending all that effort only to cancel it out with a few mouthfuls of desire? Why not let the exercise boost your weight-loss efforts, instead?
Are you struggling with or worried about feeling hungry? Lots of people are surprised and delighted to find that the 5:2 approach tends to re-educate their appetites and changes their whole attitude to food, all to the good.

I think, also, we tend to overestimate how many calories are burnt during deliberate exercise. It does help with weight-loss, of course, but is better seen more broadly, as an excellent means of maintaining overall health and well-being.

Here’s one ‘List of Calories Burned During Exercise’ that I’ve just found, and, interestingly, it gives a range of variations that apply, depending on body weight:

http://www.nutristrategy.com/caloriesburned.htm

If one weighed 180 pounds and played casual basketball (rather than a competitive game) it would take almost an hour of play to burn 500cals.
Enjoy whatever you do and I wish you much success with your goals.