Ready to start a new life journey...My Story...a little long

I'm 43 soon to be 44.  2 kids (daughter 16, son 12) Divorced about 5 years ago.  Am now engaged to a wonderful woman.  5'9" between 190 and 195.  Probably am in the worst shape of my life.  Was very athletic as a kid, even played some college baseball.  Been a smoker for 23 years, and probably enjoy my beers more than i should.  A month ago, my daughter called asking if she could have some friends over and if i would put the firepit on the deck.  Being the loving father I am, I said sure.  Took the firepit out of the garage, set it on the side of the house, opened the gate, went to pick it up and a shooting pain in my lower back dropped me.  I couldn't stand up.  I crawled up the deck stairs, my fiance is freaking out.  Of course, i figured a few ib profin and some beers would take care of it.  WRONG.  Next morning still couldn't stand.  Went to the ER, x-ray showed nothing wrong with my spine, said probably ligament or muscle strain.  Gave me some happy pills and sent me on my way.  4 hours later i could stand again.  Still sore and tight.   That scared me big time.  Now that i can move around pretty well, I thought this would be the ideal time to begin the journey to do something i've always wanted to do, a Triathlon.  I know it's going to be baby steps.   I used to ride my bike all the time about 5 1/2 years ago,  It's not a road or triathlon bike, it's a pretty nice Trek mountain bike.   But it will do for now.  I hate running, but will do it.  I can swim some, but not for any distance at all.  It's going to be a long road.  My plan is to replace my current bad habits with some good habits.  My first step of the journey begins tonight with a bike ride.  Sorry so long, but want to say a bit about where I am in my life.