You did well to give yourself 4 months to prepare for this. The more time you have the better.
Because you have an injury, and are going from a relatively unactive state to an active one, i would recommend a program which strenghtens these areas in the following way:
Use perfect form - control the weight in both directions, taking a full 3 seconds to raise and lower (or push and pull) it. Keep your body in alignment and remember to breathe. If you are in doubt as to how to perform any exercise, seek one-on-one professional guidance (even from an experienced guy in your gym). Proper form is crucial.
To strengthen your groin muscles:
3 sets of 30 reps of groin adductors. This is the machine in the gym where you squeeze your thighs together against resistance. Do the reps slowly. Relax between each rep, and squeeze your thighs together and hold it for a brief moment on every rep. Use a very light weight to start, and keep the weight light enough so you can do 30 repetitions.
Also try forward lunges, side lunges, and squats, none of which require you to be in a gym.
To strengthen the lower back:
Crunches on a swiss ball, hyperextensions on the floor, isometric holds in push up position and on your side, also the lower back ROM machine in the gym. If you feel you can handle these, move on to good morning and deadlift, but gradually and carefully increase the weight. Squats help too.
Again, form is critical. Do as many reps as you can on the body weight only exercises, and do 15 reps on the good mornings deadlifts, and squats, with a weight you feel comfortable with.
If you feel uncomfortable, out of alignment, or sense sudden pain or hear any pops or other noises, STOP IMMEDIATELY!!
You have to be careful with lower back injuries, I should know, I am rehabing from one myself.
If you dont understand any of the exercises I have mentioned, just ask.