I remember no more than a few weeks ago wanting to gain some muscle in my arms, so I began to do pushups. I also did aerobics, walked my dog pretty long distances, lifting small weights, and swinging on ropes. They were random exercises that I didn't plan really, and most of it was just out of fun/daily living. No planned "workouts". A few weeks later, my nearly non existant muscles in my arms are firm (hard when flexing), definately bigger, and noticeable. I'm a girl so I had small arms to begin with, they're still small...but I can feel a huge improvement.

Are these exercises listed above helpful? Are they what contributed to this, and is this a normal time period for muscles to build?