Most engineers who are passionate about what they do love programming and yet only a few venture out of ship anything outside of their work life.
Extrinsic Motivation is probably not a problem here.
The bigger problem is developing a critical mass where you can see something take shape.
Fitness experts will tell you that there are really two states in which a human being can exist. Sedentary and moving. The biggest challenge you face when working on fitness is moving from sedentary state to the moving state.
Once there, you will probably get addicted to working out and you will not need extrinsic motivation to hit the workout room.
Most of what you do at work, is driven by deadlines, fear, consequences and pats on your back.
Unless you have tasted the joy of owning and working on a small side project, you are in, what I call the sedentary state of the software development world. If you really want to experience the state where you are moving and loving every bit of it, get out of that couch and push yourself to build something.
You don't need years to build something huge. Just ship the first sprint.
Build just enough to have a critical mass of something which has a life of its own and an ability to morph into something gorgeous. What I can tell you, is that you wont need this blog or any extrinsic motivation to keep you working on to it.
You will look forward to your weekends, fantasize about working on your project, squeeze out hours during late night, tweak and optimize your life and even get more productive at your real job.
Shipping the first build, publishing the first few blog posts, doing the first few workout sessions, reading the first few books. The first few attempts at anything that is worth doing, are going to be pathetic, tiresome, depressing and sometimes even downright frustrating.
All I can say is, Keep jabbing.
You might not become the best boxer out there, but you might find out what you truly and genuinely love doing.