Mario needs a diet.
There's not enough room to fill up the P meter, all the pipes are out of reach and no shells are coming my way. Am I screwed? (This is in World 7-5.)
Nope! What you need to do is back up a little, probably to the left edge of the screen against the wall, hold down Right+B in order to start running, then press Down to crouch just before you get to the blocks. Mario should then slide underneath the blocks and come out on the other side.