That happened to me when I first started yakking, I had aches all over my body.....sore arms, sore *ss

(from extended sitting

), sore shoulders....etc. When I started hooping from my yak my lower back and forearms were sore the first few trips, it all went away after a few trips and it's very uncommon for me to get sore now.
I'd have to agree with Roby on the push/pull method, its amazing how much more propulsion and less fatigued you get from this technique. My buddy and I have the same yak, when we go out he can be 50yards ahead of me and I will pass him within 10-15 minutes. I out weigh him by at least 30-40lbs but can easily win him in a paddling contest.