I did not have chemo at all, but did have radiation five days per week for five weeks. There was a somewhat noticable lethargic feeling, but I was still quite active during and after treatments. The surgery slowed me down more than the radiation.
In fact, right after my second to last treatment, I drove to the trailhead and climbed the "Grouse Grind", a rather strenuous local hike that is about 2.9 km long (1.8 miles), has an elevation gain of 853 m (2800'), and is a combination of regular old forest trail (dirt or mud, with tree roots) plus 2,830 manicured steps along the way made out of piled up stone or wood.
I suppose how you react will (partially, not wholly) depend on how physically active and otherwise healthy you were before this happened. In my case, I am on a competitive dragonboat team that in addition to regular practices, also requires us to go to "boot camp" in a gym and supplement our training with some additional outrigger canoeing, so my fitness level prior to treatment was relatively high. I was also lucky enough not to have to require chemo, so I have no idea how well I would have tolerated that - it may well have been a much different story for me if I had to be injected with all that nasty (but potentially life-saving) poison.
So I count myself lucky and got the better end of it. We do have members here who did not fare as well. That being said, I felt healthy before diagnosis and now I always have it in the back of my mind, wondering if it will come back or had spread elsewhere without being detected. There is not much I can do about that, so I try not to let it worry me.