Other than winning the lottery, few things in life are attained suddenly with one big giant step! When transitioning from years of being sedentary to the wonderful and liberating life of being fit, it's important to take small steps which we call progression! Initially, use a low volume, low intensity approach and slowly, over time you'll be able to make increases in sets, reps, weight, distance, number of training days, etc.
Progression, sometimes called the Progressive Resistance Principle is an amazing thing! I've been watching the progression of motor movement patterns in my nine month-old grandson. He has slowly learned more and more complex movements by progressing a tiny bit each day. As you slowly put greater and greater stresses on your body, your body will adapt by becoming stronger and fitter.
However, if the stresses are increased too rapidly and/or with too little recovery and adaptation time, overtraining or worse yet, injury can result. Muscle growth and other positive physiological adaptations occur between workout sessions while you're at rest. Stay consistent, use progression, and get plenty of rest and you'll soon be enjoying all the benefits of FUNCTIONAL FITNESS!