Are you suggesting that if I don't drop three of those, I won't succeed in any of them? That worries me a bit, because the job part takes up LOTS and LOTS of time, far more than the other three combined.
1. Goal: Keeping fit. Invest 30 minutes each morning to that goal, which leaves 3.
2. Goal: Your job. Again, this is timeboxed until 5 pm, so do whatever it takes. Which leaves 2.
3. Goal: Play guitar. Not it gets tricky, because you could do both. But I think, again, timebox them. Monday through Friday you concentrate on your guitar, which leaves only 1.
4. Goal: Udacity. Now that you can do on the weekend. You will probably still play some guitar (I know I do), but thats not "learning", but playing (which is also important).
I think if everything is timeboxed and planned, I at least don't have problems keeping up, because I don't feel overwhelmed.
You'll know if you have so many that it's holding you back, and paralyzing you.
If it's not a problem, if you're able to progress on all of them, then congrats! No need to change anything because of a silly blog post. ☺
Also, ask yourself of those 50 different goals, which is something you HATE NOT doing? http://sivers.org/hatenot
Focus on that goal, and let go of the rest.
See my other reply here about the conditions of "flow". One is that you need to be getting immediate positive feedback on your actions. Maybe your 50 different goals aren't giving you any progress feedback, because they never get enough of your time to make progress?
Imagine kind of like some people take a 10-day meditation retreat, or a 7-day fast, maybe what you need is like a 10-day "ONLY ONE GOAL" retreat.
Pick the one thing that matters most, and only work towards that one thing. It's almost physically painful to ignore the other 49 at first, but soon the rewards from that one will be worth it. Then it feeds on itself, and perhaps could be your turning point.