No, you don’t need rigorous exercise to build muscle, what you need it a daily dose of consistent and purposeful training in order to build muscle and build it fast. Your training also has to be very specific and well diversified, so that you do not end up just building up a massive physique in one part of your body, while the other parts of you appear as though you are a prime candidate of malnutrition. Nope, not good.
One thing I personally recommend for all the people I train is that they should jog or run every day. In addition to burning fat and keeping the amounts of it fewer in the body, running also helps in building, toning, and strengthening the muscles of the calves, thighs, and buttocks. Many people don’t realize it, but running as well helps with building muscles in the abs, arms, shoulders, and backs, although there are several other more focused exercises that are designed for those specific body tissues.
If you are going to do weight training at all, you totally have to get yourself a thorough grounding in weight training essentials first. This implies that you must get yourself a trainer, but not one who does not understand your desire to keep the programs less than rigorous.
I must warn you at this juncture, though, that there aren’t too many options for building muscle without some rigor, especially if you are trying to build the muscle fast. You see, you need to push the muscles to go to places that they have not gone before in order to get them to ‘build,’ or else, you may just as well be wasting your time. However, how hard you need to push them can depend on you, what your doctor thinks, and what your trainer understands about your unique situation.
If you have a health or medical problem prior to training, you will do well to let your trainer know, though. I once trained with a young kid who hid from me the fact that he had a sickle cell condition. I ended up pushing him further than his body could take, even though he never really complained. He broke down the next day and I didn’t see him for weeks. Don’t keep your trainer in the dark, not if you want to build muscle fast.