Who cares about realism? Gta isn't a realistic game.
You don't need driving skill/bike skill at all, are you saying you can't drive without it, because you can easily drive even at the beginning of the game ?
And going to gym is something you really do only once or twice, you lose fat by sprinting around and swimming which you do anyways for missions. I haven't ever had the problem in the whole game that I became too fat for my own liking, I in fact had to fatten myself up to get this girl... The beginning HP is enough imo, prolly all but 1 or 2 missions can be finished on 1 attempt, I don't really see what the issue is, I just jumped in and raced through the missions and never worried about any stats.
I'm thinking you've played it way too little and remember things harder than they are.