Ohh..!! I guess u should focus on ur food.!! Eat healthy.. more of junk food gives u rhese kind of feelings after the age of 35.. soo start with any kind of exercise.. yoga, zumba, gym.. start with morning walks in jogers park... do some breadthing exercises in the park which will give u maximum oxygen.!! Eat food on time.. drink lot water.. atleast 3-4 lits bottle in 24hrs.. dehydration could be a reason for ur weakness.. If all this is not making u feel better,, consult physio for back pain.. then it could relate to ur spine.. Take care