I've been viewing Escapist (specifically Zero Punctuation) for a while now, and this thread has now made me sign up, only to reply to it.
I myself used to be an extremely active (though very anti-social) person when I was young, always playing sports and going for a pointless run every now and then. Come the end of high-school in '07 I gave all that up. Only recently have I gotten back into it.
I downloaded (for lulz) Katawa Shoujo to see how it was (I'm also a fan of MLP: FiM). My first choice was Emi, the sporty girl, and it wasn't until I realised that she set a challenge "1km first week, 2km 2nd" so and and so forth, that I actually got back into running.
I started off slow, 1km per day, that took me roughly 7 minutes to do. Slow compared to how I used to be, but incredibly fast compared to others of my age. I've eased off a lot since I started 4 months ago, but I can still do roughly 7km without getting puffed with relative ease.
I used to get into a lot of fights as well, and learned to defend myself because of it, getting into the National Junior Championship in Australia in 2004. Since then I've slacked off a lot, but only recently have I started that up again. I'm 1 month into my Go Kan Ryu Karate-Do training, and I've already made it to Yellow Belt 1st Karta. Slow I know, but it's progress.
I still maintain a healthy Gaming, Work and Social life as well, although, living by myself does get stressful, if you know what I mean (Bills bills bills!).
Anyway, tl;dr: Exercising is great fun, but motivation is lost quickly when doing it by yourself. Take my advice, if you're going to do it, do it with a friend or a group of people. Challenge each other to keep up. Who knows? I might even get the chance to face you one day on the International stage.