Ctrl-click on the end of the waveform so you see a yellow line on the clip edge, and drag.
If you don't want to affect the attack at all, split the clip a some point after the attack. You may need to crossfade across that "seam" slightly after changing the duration of the second part of the clip.
When done, bounce to clip to invoke the high-quality offline stretching algorithm instead of the lower-quality, real-time preview algorithm.