Sscrabble, all you can do is listen to your own body. I cycled to my radiotherapy sessions for the first three weeks, but recognised that it had become an unreasonable demand on my body after that. I remained too physically shattered for much activity for a couple of weeks after the last session, but then built up again and was hillwalking a month after the last session.
Squash is a particularly intense - and enjoyable - form of exercise (I regret I let it lapse after moving away from the courts I used to use) so you should probably be a little cautious about returning to full activity too quick. But it would be reasonable to keep it going at levels your body is happy with, aiming to return to higher intensity pretty quickly afterwards.
From my experience I see no problem with 4 miles walking (so not as high intensity unless with steep gradients) throughout treatment. You are likely to find yourself wanting more sleep than usual though!