As a newly minted diver you did very well for yourself, plus I think you learned a couple of valuable lessons:
1) Rental gear is just than, not your own. You can't trust it like your personal gear, you have no idea when it was last serviced, if it has been abused, flooded, etc. Buy your own gear, get to know it and have it serviced regularly at a shop you trust or learn to service it yourself. Oddly, most gear failures occur just after a service, and are due to the tech either forgetting to replace a part, or misplacing it, which causes the gear to fail. If you have something serviced in anticipation of a trip, test it in a pool or by another means before leaving on the trip, Standing on the dive deck, or 80' down on your first dive is not the time or place to realize something wasn't properly serviced.
2) Insta-buddies, where to begin? Some are great, knowledgable divers...and some are not. As a certified diver you know that your personal safety is dependent on you, and you alone. A good buddy can help you learn much during a dive, or provide some guidance around to boat / dive operation; a bad buddy, can ruin your dive and maybe even place you in danger. Your safety is job one! You are the person who is in the best position to know how you feel vis a vis your safety. You did right in monitoring your air consumption, heading back to the boat with enough air to complete your safety stop and surface. I more observant buddy would have talked to you prior to the dive, ascertained that you were new, and been nearer to you from splash to the end of your dive.
Having competent buddies, who are willing to not only dive with you as you learn the ropes, but who are willing to help you progress and learn many of the secrets of diving are a real boon. That said, they are not everywhere at all times. Join a local dive club, or hook up with a specific operation [LDS or maybe a resort or live aboard you can frequent] and get to know the folks there, in almost all cases you will find a mentor who will gladly help you progress from newbie to mossback.