Personally, I found focusing to be far worse than not focusing. People don't do resolutions because if they did them, then life would change. Not necessarily by a lot, but something changes. If you had finished building it in February, you'd have to figure out why it hasn't had one paying user. And now that you've built the app, you can still bring the resolution forward next year, but hesitate to.

So I find the lifehack is to have two targets. You procrastinate on one then do the other. You take baby steps to avoid selling that app and end up doing the half marathon.