I agree, my take on this is that a guided trip will cost *far* less than say, a broken leg or the like, if things go wrong, even if you have good insurance. Things might go well, they might not.
I first saw, and started watching that mountain when I was about your age, and finally summited it on my 61st birthday, using International Mountaing Guides in Ashford for the guide service. That was money VERY well spent. And even though it was a most perfect day, weather wise, had I tried it on my own with some "online climbing buddies" of unknown capabilities, I can think of a few times things might not have gone well at all. I learned a lot that I didn't know that I didn't know by going with guides.
Prior to making the decision to go, I read a LOT; (Freedom of the Hills, various other books, nwhikers.net, Cascadeclimbers.com, Summitpost.org), and took a few trips up the trail to Camp Muir prior to making the decision that yes, I can do this, but with someone who knows HOW to do it.
I have no doubt whatever that you can do it. I did and I'm not in the same conditioning category as you appear to be, but there is more than physical conditioning and good weather to make it up and (more importantly), back down Rainier. That is a serious undertaking.