64oz a day is actually what is recommended for everyone. Eight 8oz glasses. For a normal person with an average activity level. But always keep in mind, if you are sweating, you should be drinking!
The primary functions of the large bowel are electrolyte and water absorbtion. The end product being poop. It's the poop factory. If you are constipated, the first thing to look at is adequate fluid intake. For ages it was said the colon extracts all the water. Now newer publications are amending this and say the colon absorbs MOST of the water but there is also some fluid absorbtion in the small bowel. We have the same colon we had preop. No changes there.
This like so many other things is individual. There are DSers who seem to need more fluids than average people and some who don't. Immediately postop quite a few people can't or won't drink enough. Probably because it's uncomfortable. That's why you always hear walk and sip. Over time you will find out what you need. You pointed out your BUN, good indicator. Plus you can tell by looking in the toilet. Your urine should be light yellow, sort of straw color. However, if you take loads of B's it may be bright yellow! B's are water soluble and if you take more than you need, you lose the excess in your urine.
I have never been a big water drinker. I drink coffee, tea, and fake soda. But I am getting enough fluids. Never had any problems with dehydration at all. I always SAY I'm going to start drinking plain water but I never do it! Somehow one of those little flavor packets always jumps in the bottle! Personally, if it's liquid and I drink it, it counts. By those standards I get quite a bit more than 64oz per day. The purists say if it's caffeinated, it doesn't count. For me caffeine is a constant so no real issue. And caffeine is a SLIGHT diuretic. If caffeine was going to dehydrate me, it would have happened looooong ago!
Google all this and read about it. Lots of good info out there.