i know there's a lot of talk out there about over training, but that doesn't really seem like anything you need to stress yourself out about. you probably shouldn't exercise all day, every day, but if you enjoy working out then do it. personally, i love days when i'm "exercising" all day because i feel like doing my normal workout, going for a run, and then helping somebody move. i almost look at it as play more than anything else, and that is an often neglected part of the primal blueprint.
regarding going heavy, just push past the fear. programs like start strength and stronglifts are great for easing you into heavy lifting while building strength (including grip strength) along the way. if you do make the switch to lifting heavy, it's likely that you will naturally back off some of the exercising just because it can take so much out of you, but that won't be for a little while if you follow one of those programs. as fun as medium weights can be for an energy boosting workout, you can hardly imagine what it's like to hit a new personal lifting record with heavier weights. it feels amazing.