Wish you the best with your detox, you can do this and good on you for making the decision to quit. By all means it the ground running and make use of that gym.
The best advice/experience I can share is that if/when early in your quit you feel you want to drink and feel like you miss it terribly, it does Not mean you have not quit or that the time isn't right . It may/will be the addiction struggling to hold to its pathetic existence , the one we facilitate, the one that will eventually kill us. Stay resolved , choke out the last gasps of the addiction, it will be trying every mind game in the book , don't fool yourself this is the right way forward , the best decision. Experiencing the wanting/desire/urges/cravings ect are not signs of not having quit, stay resolved , be strong, you got can do this. In time, you will hopefully look back on this thread , point it out to fellow travellers astonished at how strong the hold of addiction was.
Time takes time, a very poignant and true phrase I have seen here on SR, and so true , stay resolved through that time and be ready for the 'it gets better'time , because that part is true , too.
wish you well