First off, I hate gyms. I find that they can be noisy, stinky, intimidating, basically everything my Aspie brain hates.
When people commit to an exercise program, they are always over-ambitious and unrealistic at first. People don't realize that rest days are just important as workout days. The thing is that after a workout, your muscle tissue will sustain minor damage and you will feel a bit tired, physically and mentally. This is especially true if your body isn't used to physical stress. The mistake that everyone makes is that they go too long and too hard for those first few days. If your body doesn't know how to process all this unfamiliar stimulus, you will start feeling the symptoms of over-training: no desire to exercise, moodiness, muscle fatigue. Your body is trying to tell you to stop, and this is of course when alot of people fall off of their training plan. If you really want to stick to a training plan, make it really light at first. For example, you can maybe do two days of cardio and then one recovery day. Then you can very gradually increase the length of your cardio workout as the weeks pass. In general, your training plan should increase in intensity for about three weeks, then use the fourth week to allow your body to recover. You will feel much stronger and healthier after the recovery week. Just remember it's very important to "pepper" your exercise plan with rest days for every 2-3 exercise days, otherwise you will be more likely to quit.
Also, if you're trying to loose weight, it's worth the money to purchase a continuous heart rate monitor (the kind with the chest strap). It's a great way to measure exercise intensity. I don't know your particular fitness level, but I would keep it at light cardio for now to build a foundation. Then when you are feeling more confident after about 2 months, you can work on increasing muscle mass and your lactate threshold.
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Here's to the crazy ones. The misfits. The rebels. The troublemakers. The round pegs in the square holes. The ones who see things differently.