yea u dont want to lean back in the run up, but you do want to get more upright.
My ideal run up is -
Get back to your mark, take a few steps back, then sort of jog in with very little steps to gain a bit of momentum, then as soon as you hit your mark immediately get into full strides, arms pumping with short pumps to push you forward, this is in a sprinter's run mind you, so you're slightly leaning forward. Then when approaching the crease, start to get more upright, so when you jump, you should be upright or slightly rocked back, which allows for more pace and bounce. If you dont get upright, you will release from a crouched position, with a bent knee.
Watch Brett Lee and Dale Steyn for ideal run ups, you will notice nearly every bowler, if not all straighten up in their run up from the sprinter's run into a more upright position, which allows for a bit of rockback during the jump.
Run up is one of the most important parts of a successful action, and it is neglected by so many bowlers because they think the action starts at the bowling crease, which to some degree is true, but you want to look at it as a whole. The action should start when you hit your mark in the run up, it finishes when you finish your follow through.
Just remember, the run up should be SMOOTH. If your run up falters, then your jump falters, then your load up falters, then your release falters.