I've done classes, because like you I'm not disciplined enough.

Think about the proper forms of poses.

Definitely at least start with the classes. While I don't do Yoga, I always struggle to get myself to incorporate Core into my workouts. So I joined a new gym that incorporates core into all the classes so that I am working out Core at least every other day. If the classes are too expensive one place, look around you will probably be able to find a cheaper price somewhere else.

Alternatively, you could try to see a chiropractor. I see one for all kinds of injuries both sports related and not. They're not that expensive and they know a lot about how to help muscle/ ligament/ tendon issues

As a personal trainer, I'd say if you're only going to be going a couple times a week for a more extensive workout I'd recommend at least 3 days per week. During such workouts, I'd recommend a short run to warm up and maybe cool down but the majority should be weights. If you are looking for weight management than resistance training (weight training) is the best thing you can do for yourself. For every pound of muscle on you, your body will burn an additional 50cal every day regardless of whether you workout or not. So when you build muscle you're actually helping your body burn fat even on days when you can't make it to the gym. While cardio feels like a great workout it really doesn't have the long-term benefits of resistance training. So while you will be burning calories while you run, as soon as you stop, so will the burning of calories, but this is not so with the resistance training. Best of luck out there!

I go through this same conundrum all the time. Honestly just go. Get exercise whenever you can. And if you can get in a harder workout, even better. But each day is a new day and the challenge presents itself all over again :|

My personal opinion is that something as simple and obvious as taking the stairs instead of the elevator can help if time isn't an issue and your goal is just to lose a few pounds. If possible, carve out a few evenings a week to go to the gym, but don't sweat it if the best you can do is get a brisk walk in.