After unlocking the Ganton gym, I jogged from the house to the gym to raise my Stamina, entered the gym and began pumping irons to raise my Muscle and after I hit the limit, I jogged back home to save. I repeated this process about 20 times before my Muscle was maxed out, my Stamina nearly hitting the limit.
After maxing out my Muscle, I borrowed the BMX in front of Loc's house and raised my Cycling Skill because...why not? After (I think) 9 in game days, I had maxed out my Cycling and Stamina skills. Hell, yeah.
Then I went to work on my Lung Capacity because I know it will come in handy in one of Woozies missions. Still working on Lung Capacity, it's really hard to raise.
There's more for me to do in Los Santos than there is in the entirety of GTA IV, gameplay-wise, not story-wise. Suck it, Niko.