As so many others have written, it all depends on the individual. The amount and type of activity that is helpful for an ME/CFS patient depends on so many factors -- how long you have been sick, what your symptoms are, whether you have a support system if you crash (e.g., you need to be more careful if you don't have someone else who can buy groceries when you're too sick to go out), and so on.
When I first got sick 20+ years ago I was much worse. I had to rest a lot more than I do now. I also had to learn a lot of lessons about what activities were helpful and which ones to avoid. For me, not pushing myself was one of the hardest lessons to learn.
For example, for the first 29 years of my life, when I was healthy, there was pretty much no payback from pushing myself physically other than maybe some sore muscles the next day. In fact, pushing myself in the gym (lifting heavier weights each time, running a bit farther each time, etc.) was a
good thing. Pushing myself, assuming that I also got good nutrition and rest along with it, was how I trained my body to get stronger and faster. I loved hiking and going to the gym. I also enjoyed my job and looked forward to going to work every day.
Then, after I got sick, I suddenly had trouble just doing normal activities of life. As someone else mentioned in this thread I had to lie down after taking a shower and before getting dressed. Then I couldn't even stand long enough to shower and had to use a shower seat. I had to sit down to do pretty much everything because standing was so difficult - nausea, dizziness, etc. My onset was not overnight but after a few weeks I was taking sick days every other day and very quickly had to go on full-time medical leave and was mostly housebound (thankfully my husband was able to provide financial/emotional support and take care of household tasks).
To make a long story short, years later I discovered that a large part of my problem was a type of Orthostatic Intolerance, Neurally Mediated Hypotension (NMH). Symptoms of NMH are made worse by standing, heat, and several other factors. See
http://www.cfids.org/webinar/cfsinfo2010.pdf for lots of details.
Those first few years that I was sick I did not know about my NMH so I did not know what things to avoid that would make my symptoms worse. Once I was diagnosed, and started treating it (water, salt, fludrocortisone, midodrine), and avoided triggers (heat, standing, walking on a full stomach, etc.) I got a little better. Now, a bit of walking, only on my good days and only when it is cool (or inside an air-conditioned building), interspersed with lots of resting, is what works for me. Also, lying flat during the day is not as good for me as resting in the recliner, legs elevated but head upright. And sitting in a chair, with my feet down, does not count as resting, because this position can aggravate NMH. Before I knew about this problem I used to think, I'm sitting completely still, not doing anything physical, so why do I feel so crappy (nausea, exhaustion, brain fog) after a few hours? Now I know that sitting still like that can aggravate symptoms of NMH.
Also, the other thing that seems to work for most ME/CFS patients is to rest enough so that you start to feel better *first* and then do some activity. Don't assume that the activity will make you feel better. Of course, it may depend on the patient's personality but pushing myself has never worked for me. I have to restrain myself from doing too much. I love to be active and I
hate to rest--so much so that, even though I know better, at least every few weeks I end up doing a little too much and have at least a minor "crash." But after I "crash" resting always makes me feel better.
I hope this is helpful. It's so hard to give general advice when patients are all different. The most important thing is to listen to your body. The things that are helpful/harmful for me may be completely different for you. Best of luck to you!