On a serious note, to build up muscle mass it may help to eat some protein supplements.
These usually come as a powder that can be stirred into milk for drining (tastes like crap, though), though I've seen them in candybar format as well (probably tastes like crap too, never tried em). This can help as it can be quite hard to actually eat as much meat etc. as is needed for fast weight gain if one is not used to devouring vast amounts of food every meal. I personally can't gorge like that regularly without getting sick, so a protein supplement was what I needed to help me gain a few kilos in a reasonable time.
Excercise is still needed, of course, but not *that* much. No need for dedicated body building or long distance running if the goal is simply to build up some extra muscle tissue in preparation for what's to come (which I think is a really good idea, chemo can leave people quite skinny so a preemtive strike against that makes excellent sense). 20-30 minutes twice a day, 3 days a week should really be enough if starting from the level your average non-athletic person is at. Focus on weight training (or whatever, what you need is resistance, doesn't have to be weights), not cardio, as the latter really doesn't build muscles very much at all (it helps though, as having a strong heart is just a good idea regardless of what you're getting into).
Run sequences focusing on a single muscle group at a time using a weight where you're able to lift it about 8-12 times in succession on the first run. Then take a few minutes break, and repeat until once again you can't lift it any more. Do this 3-4 times, maybe more if in good shape, and then switch to a different muscle group and repeat. If this is done consistently and the protein intake is up to the needed level, weight and muscle mass will start to arrive fairly quickly, and it should be possible to tell a clear difference after a month or so.
Remember to warm up or much pain and anguish will ensue, and be careful not to overstretch when stretching after the excercise
