Sorry, been away a bit.
Good for trying. I think if you can power through the first 30 days you'll stick with it.
Strength can fade over time, esp with things that we don't typically do on a regular, daily life basis like pull ups.
We're all pretty familiar with the traditional targeted exercises: bench/shoulder press, push/pull ups, squats, etc. Me, much better at some, worse at others and some I just hate to do. (yeah, I said that!).
But finding what excites you is gonna be key. Running? gym? DVDs? Fighting? Cardio? somewhere in there should be something that you're particularly interested in, so that even on the low motivation days you can manage to bang something out.
For me, some stuff produces more immediate results. Chest exercise for example, seems to show more immediate results than say squats (which A. I don't really like nor am very good at, and B. unless done regularly seems to be stuck in same place, and then we're back to A.). haha!
I think I've found what works best for me in cardio/fighting (see Les Mills Combat, KickboxingHeavyBagWorkout.com, or Krav Maga). Seems to rip up my body while doing 'something else', and seems more practical, every day, endurance strength.