Sounds like you've already accomplished lots, Tasha! Weight loss, getting jazzed about cycling, daily rides, increasing the distance, setting distance riding goals...and, of course you can do the STP! It's totally possible as you've already shown yourself what you can do with determination and planning. There's a terrific and supportive community here to help cheer you on.
Also, locally (all over the Puget Sound area) there are tons of supported rides throughout the year, along spectacularly beautiful routes, so that you can vary your training if you want and build on the mileage/duration of your rides as you prep for the STP. It's always fun to set goals and then amaze yourself, eh?
Glad to hear that you got your saddle adjusted and are happily feeling the difference; sometimes you only have to move it a touch. I'll echo the full-on endorsement for cycling shorts - and remember, no panties under! A bike fit is key in avoiding pain due to incorrect body mechanics/positioning. I'm not familiar with any LBS in Tacoma, but am sure that they're there for you to find; you can learn tons from the helpful folks in a good one.
Happy Trails!
Mary