Randy,
I have to reinforce what the others have told you about eating and everything that you want to, and then some! Three weeks into your treatment eating will be become a real chore (if you're like most of us) and by the end of treatment you may have a naso-gastric tube or a PEG and be struggling to take anything by mouth. Most of us lose weight so if you put some on prior to treatment that's not a bad thing.
Now that I'm unable to eat solid foods I still remind my daughter that she stopped me having a burger in the week before I started my last treatment. (You shouldn't develop swallowing issues - I have them only because I have had radiotherapy twice)
I agree that light exercise is a good thing throughout treatment but do listen to your body! If you are tired sleep. Don't try and overdo it!
Good luck and keep us updated with your journey.
Sue