Kathy,
Give yourself a break. At 6 weeks out I was still a mess and hardly working at all. I mostly sat in my easy chair and read books or watched TV, made brief trips to the office occasionally to read mail, and took a daily walk. Other than a few errands that was it. As you said, radiation is very strong stuff and it definitely takes it out of you.
I am now 7 months out of radiation and I still don't have near the energy that I used to and still don't work full time. For one thing when you cannot really swallow food and have to depend on the PEG, there is no way to get enough nutrition to be high energy. I still have mine and use it about once a day. However many people do get rid of theirs a lot sooner than I have.
So, no need for guilt. This is your time to convalesce. Work when you have to or really want to. You will feel better in time.

Take care,
Danny G.