The same night that I posted that, I went to a bar and randomly ended up talking to a local san diegan MMA fighter that was trying to order a beer next to me, and he was beastly lookin'. I got a Ballast Point victory at sea 2008 coffee/vanilla imperial porter, and apparently it's his favorite beer, so we started talking about it, then eventually we discussed his training and how he got started lifting and fighting.
It turns out he was a twig for the majority of his life, then after high school he got fed up being the skinny guy in his group of friends, so he started lifting like crazy. He said he could only take 1 rest day a week in the beginning, and he usually worked the same muscle groups at least 2-3 times a week because otherwise he'd lose his gains too quickly.
He seemed to know what he was talking about, and his own results were proof enough of that, so I told him about how I'd been doing p90x, and he kinda chuckled for a sec. I asked what he thought about it for putting on muscle, and he said the workouts are a good guideline for starting out to get familiar with what exercises work different muscle groups, but overall, the program isn't designed to get muscle mass quickly.
His suggestion was to take the exercises I learned from the p90x, and then add as much weight to them as possible, so I can only do 3-5 sets of 5 reps until failure. Also, for my legs (which he gave me shÃ*t for lol), he told me to skip the p90x workouts for legs, and to focus mostly on squats, lunges, and calf-raises at least 2-3 times a week for a month or two until the gains slow down, then to add in other leg exercises.
I've only lifted twice since I talked to him, and by following his suggestions and changing my regimen, I'm more swole and feeling much better about my progress. When I was following p90x as intended, I never felt like I'd worked out the previous day because my muscles weren't sore or feeling "worked" at all.