I've had a gym membership for behind my apartment for 2 months now. Tomorrow will be my first day going. I got the membership to remind myself that I have to do it. It's working.
By devoting 10 min 3 times a week doing some high intensity intervals you can achieve a very good level of fitness. Even better, it won't require a monumental amount of mental effort (like e.g. long distance running does).
I recommend you do this on a machine whose exercise is low impact for your joints (either a Concept2 or an Airdyne). These can be found very cheap on Craigslist if on a budget.
Add in some free weight exercises (dumbbells or kettlebells will do, a pull up bar also recommended), some abs and some dynamic stretches to have a really sound training plan.