the hardest part is always the first step. since right now your feeling unmotivated and you dont have energy, the first thing your should do is set up a good diet. once that is up and running, you can get back into your old routine. dont expect to wake up one morning and suddenly feel like working out and train everyday till you are where you want to be. start of small. go for walks. then runs. then start some resistance training. then suddenly you will realise that everything is falling into place.
as for the injuries. avoid things that might jarr your knees or joints (e.g. hard surface sprinting), but otherwise it shouldnt be too hard for you to build up a foundation for you to get going (since you have a good history of exercise and keeping fit).
as for motivation, it is hard for anyone to tell you to do it. it should be enough for you too look in the mirror and be self-conscious and depressed and want to change your lifestyle. also think of your son when getting back into it. you wanna run around and play with him and do the things he wants to do.
if you have any specific questions feel free to ask.