The hardest part of any habit is the first minute.
Make the first step tiny
So make the first step tiny. Put your running shoes on. Open the book. Write one sentence.
Most days, once you've started, you'll keep going. On the days you don't, you still kept the promise, and that's what turns into a habit.
Day one in Plan90
Plan90 does this on day one: one small task of about ten minutes, so the first tick comes easily.