If you know that you will be challenging yourself and searching out steep hills, learning to stand and pedal is not just about helping reduce the fatigue of the sitting/spinning muscles, but sometimes you will be FORCED to stand because the grade is so steep that even your granny gear won't allow you to sit and spin, and standing while pedaling at a super slow cadence (30 rpm) is the only option if you don't want to walk up the hill. And there's nothing wrong with walking up when you're starting out, and your fitness hasn't matured yet. I recall a 50 mile ride I did early on with lots of ascent where I had to dismount and walk up the final 200 yards of about 5 different climbs that day. 2 years (and 15,000 intense miles) later I did the same ride and didn't walk once, and in fact, I couldn't even recognize which hills caused me to dismount and walk, since I was hammering up everything.

I also lost a lot of weight in my first 18 months of cycling (228 down to 168, and 15% body fat), but the lifestyle I was actually able to sustain for the long-run caused me to drift back up to 175 and stay there (with the help of ~5k - 8k miles annually).