I've been going to the gym somewhat regularly over the last ten to twelve years. Over the years, many people I knew decided they wanted to loose weight/get in shape/build up muscles blah blah blah... I usually think of these as New Year commitments and gyms are never fuller than in January. :x
The truth is, for most people, ironically including me, gym is NOT fun. It's really just about forcing yourself, making it a habit and sheer determination. Hence most people who I knew who started at some time stopped at some time, usually within the first 3 months. Typical reasons: doing way too much right from the start and then feeling like total cr@p/having unrealistic expectations about getting a bikini figure/sixpack in 2-3 months.
And the other thing is that after 3 months you will notice nothing, except maybe feeling a bit healthier. After 6 months you might see something. Worth the effort? Probably not. It is only after keeping fit over years that it will pay off one day in you looking better than the typical flabby guy your age (who does nothing in his life except handling the remote control and chugging down beers in the pub on Friday evening) and having a healthy heart & metabolism. I do it mostly for the latter. I'm surrounded by rather disgustingly fat people at work, that helps keep up my determination. :) If that sounds harsh... you haven't seen them!
You must think of it as a long-term thing and should not expect it to be fun, more something that you have to do for your health!