I admire your desire to start an exercise program even while you have all the responsibilities of a new child. I guarantee you will see benefits if you can keep it up over time, even if you can only do a little. The important thing is just to get some activity in for a few hours a week.
My current program is to run 3x a week and lift weights 4x (mainly squats, dead lifts, bench press, overhead press, barbell rows, pull-ups, and dips). After the weights, I’ll usually do some time on the exercise bike or take a long walk, just to have a little cardio every day. My usual run is about 4 miles, though they range from 3-6.
I think the couch to 5k program is outstanding, but it could be hard depending on your current level of fitness. Even after finishing the program (which is basically how I started), I still had a lot of difficulty running a 5k without walking for part of it. It took several months before I got over all of the beginner’s pains, but I was extremely out of shape when I started. Even now I wouldn’t say running is easy. There’s no doubt it’s a demanding exercise, which is part of what makes it so rewarding. I always enjoying finishing a run (!), but I would say 33% of the time the run is really hard, 33% it just feels like routine, and 33% I feel really good.
But I don’t have any concerns about injuries. There were times during the first several months when I had pain in my joints and limped around a bit, but now I just get what I consider normal soreness. It’s certainly possible to over-train like your boyfriend, but millions of people all over the world are able to get themselves into running shape, and if a person trains right, I think it should be protective against injuries, at least for hobby runners. There are plenty of studies that call into question the idea that running is bad for your joints. Sedentary people get joint problems all the time too.
As for diet, I mainly eat what gives me energy and makes me feel good. I slow down considerably when I don’t get enough carbs, and I don’t recover as fast if I don’t eat enough protein (for me, that means at least 100 grams a day). If I’ve burned a ton of calories, I might feel like I need a pizza and beer. Other times I eat more moderately. As long as you don’t go to extremes, you should be fine.
I’m excited that my thread was motivating to you. I think you’ll do great!