Hi there and welcome... I'm assuming you have base of tongue cancer... not oral tongue based on your reference to HPV, and the lack of surgery....

I too used to exercise a daily, and tried to maintain this until about week three. I am now back at yoga (though I've exercised on and off for the last 8 mos.) I'm not as diligent as I used to be but that's because of my diet. I eat no bread and no sugar not animal product and very little processed foods. Because of this I'm at the low end of my weight scale.

But do it as long as it doesn't compromise your health... Gyms are full of germs, biking is definitely better, but at some point you may not have the energy. One study I did read was that people who walked at least once a day (I walked my dog twice) had a better survival outcome. I think it has to do with O2 intake. Good luck - it wont be easy but you'll get through it.