Tired: On immunotherapy and I want my strength back

I was diagnosed with stage 3 lung cancer almost three years ago, had a lobectomy and chemo. It came back in a year a small spot ,radiation and chemo. Scans just show scaring
Taking immunotherapy now have for almost 6 months now so far so good. But im so tired! Some days I can barely get out of bed. I want to get some strength back. I go to the gym and walk so far i can manage a half a mile.
First why am I so tired? It's been 7 months since the treatment. And more can I do to get some strength back?