I actually don't do anything to keep fit or exercise, I ride a Bike simply because I love to ride, the benefits, health-wise, are a coincidence...
I used to be a semi-pro Footballer (Soccer Player) and I only did that because I loved to play, I went to a Rugby School and hated that because I was always getting slaughtered.
I guess I developed good Legs through Soccer, the running up and down of Stadium steps was sheer misery during training but I think it paid off...it came to the point that I reached 40, knew where I should be on the Field but couldn't get there fast enough, I started to have problems with my Knees and Ankles, so that was that :down:
I started Cycling again back in '03, I hired a Coach and looking back, I owe Him so much, even though I hated Him most of the time he was my Hero, it didn't matter how fast I got I could never keep up with Him... :-S
I love to ride in fast Groups, it stretches me but more than that, I dearly love to help, encourage and watch those new to the sport, the greatest gift to me is to see them eventually pass me on a Climb or a sprint and see them smile from ear to ear, it's fantastic!!
One example, last year, I took an 18 year old Kid under my wing who had grown up with a muscular disease, the first challenge was to make him ride in a straight line, evntually he got faster, and indeed straighter, I saw Him on the Backbay a few weeks ago, he was with a Friend who looked wiped out "He's so fast, I can hardly keep up with Him" his Friend said. That was brilliant...
Sorry, I digress, I think the only way to go is to find something you really like doing and possibly a group of People or a Partner you like doing it with...yeah, I know, I know... 8-/
So what do you like doing? Running? Walking? Swimming? I guess that's the first step.....