Kudos on going Babywise. My good friend, who had two kids before i had one, made me read that book while I was pg, and then called me repeatedly to help and support me. We did babywise with a little bit of Baby Whisperer thrown in with my son (which helped with understanding the cries and knowing whether to go in or let him CIO).
It IS hard to leave the house especially in those early months when they are taking 3-4 naps a day--it's gets a bit easier once you switch to the 4 hour schedule--which you do around 12-15 weeks if I remember correctly. Trying to time outings while she is awake, or doing like you do with church which is to just go and get back on track the next day are really the way to go.
My advice is try to keep the routine on track, but don't beat yourself up over it when you get a day that isn't perfect. I stressed over it sooo much--which was really unnecessary. Look more at the order of things (sleep, eat, awake) and setting a consistent bed time, more than fretting over them waking up 15 min early from a nap, and then wondering if you should feed them right then or wait 15 min.
If you do have a lot of errands to run in a week, then just give yourself a morning to do so. Keep the feedings on track as much as possible and let her sleep in the car seat if necessary (and if she will). Just like with your Sundays, that day may be off, but you can get right back on track the following day. the key is too not have too many of those off days so that it confuses the routine, if that makes sense.
Also, get your husband to run errands too--especially in these early months--picking up things on the way home, etc.
Finally, just to plug the whole routine method like advocated in the Babywise book (and now others too). We are a total success story. My now 2 1/2 year old was sleeping 11-12 hours a night through by 16 weeks (he would still wake sometimes and still does but can usually get himself back to sleep unless he's sick or something). We had a hard time with naps at first too, but by 5 months he was a great napper and still is. My toddler rarely objects to nap time or bedtime--he goes down very easily--so those early weeks were soooo worth it especially when I talk to other moms of toddlers who are not on a nap/sleep routine...
Good luck and hang in there. Once she goes down to one morning nap, one afternoon nap and an mini nap in the early evening, you'll find time to do things in between naps, and once she goes down to one nap--you won't know what to do with yourself in the mornings!!