I think your chances are actually good. I did my first hike in similar circumstances. I was early 40's, and a bit overweight (so you should have a good advantage) , but in OK condition. Did nothing to acclimate. A VERY difficult hike, but did make it. Mostly "mind over matter". I've summited 3 times since, once via the Mountaineer's route. All included much more preparation. A lesson well learned:-). I was hiking with a friend and that was certainly an advantage. You should have no problem hooking with with other hikers. I've always found hikers to be very friendly and supportive.
I like your plan, except I would not recommend the Devil's Postpile as your warmup. There is some debate as to best strategy for acclimation, but the one I find works the best is: Hike high, sleep low. This is "relative high, and low". For sleeping, 8K' is OK, 9K would be better (Mammoth Mountain Inn is at 9K). Instead of descending down to the Devil's Postpile, simply hike up Mammoth Mountain. Go as far as you like (top is just over 11K'). There is a trail, or you could simply hike up one of the ski slopes. Be wary of the mountain bikes depending the bike trails.
Also, on Friday, upon arrival at the portal, I would recommend hiking part way of the trail to familiarize yourself with the trail during daylight. The next morning, you will be starting in very dark circumstances. No moon. I suggest you investigate the
"old" trail. It is not really a trail, but more a "boot track", but not difficult to follow. It cuts a bit off the distance, but also avoids one creek crossing.
Providing weather does not intervene (it can change quickly, be aware of it), I think you will make it.
Look forward to your trip report.
