Not to mention you have to keep your breathing intact. When you go back to the starting point, You'd start off with a big breath-in and breath-out and keep doing a breath-in and breath-out in sync with your running speed and while you're about to release, grab in some breath and let it out just as you bowl. That should add up a little power to your delivery, and the surprising thing is that you'll have a semi-automatic control on your line.
Work on the lungs, they're gonna play a good part on your body.
I also agree the action needs to be fluid-like; smooth. If you check out some of the Athletes, you'll find a intense development of this movement. Be it gymnastics or Athletics. Rushing in is not pumping into the crease.
Secondly, Your spine will depend on the bend of your action i.e. It will depend on how you bend out after you bowl and go into the follow through. You're gonna have the peak of the stress falling on your back when you just release the ball and are about to go into the follow through. Imitate that part by standing still, check where the effort is going into at the back, assuming you have a coach, and you can get some adjustments in the way you bowl.
Thirdly, Fitness is absolute. You should do as much as you can to make your muscles fit. Specially your lower back and the back of your thighs through your hips. The regular excercises before the game or before you bowl should help you.