Argh. Just wrote a response and it disappeared. Second try:
I'm sorry your kitty is having trouble pooping. This seems fairly common in older kitties. My Celia had chronic issues and got Miralax every day. We started with 1/8 tsp daily and increased to about 1/4 tsp. over 4 years or so, when the 1/8 tsp no longer seemed effective.
Anything you can do to get more water into her will help. Adding water to her wet food is great. You can put in quite a bit and mix it up so that the food is the consistency of porridge or a thick soup.
Have you noticed that she's drinking more water or peeing more? Since she is a senior kitty, it's possible that she's starting to have kidney issues. Don't let that freak you out. Even if that's what it is, many CKD kitties remain stable and live for a very long time with the right treatment. Celia was diagnosed about 16 months before she passed away, and had she been the type of kitty who would have tolerated more treatment, she'd still be with me now. Unfortunately for me, she wasn't. She would have hated it.
I do hope it's not CKD or anything serious. I'll keep my fingers crossed for a good vet visit tomorrow!