Pre-Cooking Steak in a bag, Any suggestions?
You've basically re-invented sous-vide cookery for steak.  Steak is about the easiest food to cook sous-vide.  You can find a lot of resources describing it on the web, but here's a few notes

1) 170F for 2 hours undoubtedly left your steak very well-done.  Not a problem if that's how you like it, but also not necessary.  You can get medium rare by cooking in the 130-135F range for two hours.  Cooking sous-vide below 130F (necessary to get steak rare) is not recommended for safety reasons, as temperatures that low can encourage the growth of dangerous bacteria.

2) With sous-vide, there's no need to rest a steak after searing it.  Resting grilled steaks is necessary to give heat time to move from the outside of the steak to the center, and actually get it cooked to the desired temperature.  With sous-vide, you already did that in the water.

3) With an impromptu sous-vide setup like yours, you probably don't want to cook for more than a few hours.  You risk your temperature getting too low overnight, and then you're down in the bacteriologically dangerous ranges.   You can get some great effects cooking longer (I had 24 hour marinated skirt steak last night that was amazing), but you should probably do so with powered equipment.  There are home sous-vide machines available (a bit pricy) or you can build your own.  There are dozens of different rigs described on the web.  I built mine for about $100.

4)Longer cooking times are better for tougher cuts anyway.  If you're just looking to do ribeyes and strips, then two or three hours is fine.

5)You can marinade, but obviously not in supermarket bags (unless the supermarket marinades, e.g. pork tenderloin).  To do so, you either need to purchase a vacuum sealer (not too expensive), or be clever with drawing air out of ziploc bag and sealing it very tightly.