Lots of good hotels recommended, but wanted to point out that the OP didn't mention when she'd be in Bangkok, and also mentioned that she didn't mind walking everywhere. These are actually pretty important points for first time visitors.

OP - I'm a big walker as well. It's not uncommon for me to walk 5-10 miles in a day checking out a European city that's new to me. Bangkok is different. It's huge. Nothing is as close as it looks on a map. It's hot and humid. And it can be rainy depending on when you're going...and not a rain you want to walk too far in. I find myself taking public transportation regularly there.

While I think you'll probably be happy in any of the Bangkok Marriotts, I'd strongly suggest you stay near a BTS station (the skytrain). You can still walk if it's a nice day or you feel like it, but I think you'll appreciate having it at least a couple times during your stay. Really, any station is fine. The Renaissance, for example, is as close to a station as you can get. But preferably you'd be within a 5 to 10 minute walk. I've only stayed at 3 properties there, so I won't direct you toward a specific hotel. But worth considering as you look at hotels.