> I think for myself, the key is knowing that the guy who biked across India wasn't writing a novel. Or at least, he wasn't biking across India, writing a novel, and maintaining a 40+ hour job and raising a family.

Well exactly.

A bit of back-of-the-envelope calculation tells me that (to sicken the GP) I write at least 30000 words every 5 months - and that's with a time investment of 3-5 hours on a Tuesday evening (I write commentary for a weekly paper). I could hit eighty or ninety thousand if I didn't like video games and talking to my girlfriend etc. There's no magic there - my dad is a writer, and when I was 6 I wanted to Be Like Him, and ever since I've enjoyed writing. You do that for 22 years, you pick up some speed and style.

On the other hand, whenever I think "I need to contribute to $OPEN_SOURCE_PROJECT", I suffer immediately from 'don't know where to start' paralysis, and usually give up pretty quickly.

Nobody's ever going to be confident at everything.

EDIT: The other thing is sort of 'confirmation bias' - stuff you're very productive at feels very easy, so you don't notice the productivity so much as you feel acutely useless at things which feel difficult.