Hello,
Muscles follow the principle of "use and disuse".If one uses the muscles they develop and waste if not used.
The exercises you are doing are fair enough and I would advise you not to put undue strain.You would need a high protein diet,calcium and water.Protein diet helps in building the muscles and water helps in flushing the toxins whcih tend to build up while on high protein diet and exercising.I do hope you are not a hypertensive and donot ahave any kidney problems. If you have them,you would need a dietician's advise to guide you.
The muscles of the thigh insert in the tibia and fibula and one needs to work out these too. the best form would be what you are already doing i.e. walking. Hope this was useful.