It’s easy to let yourself slip (and for some never get started). I let it slip for 15 years, and I tell you now what you already know, it’s easier to maintain then get back in shape and it’s easier in your 20’s and 30’s then your 40’s or later. Stop wasting time and get moving – it’s never too late!
Everyone’s path is different, but my recommendation is to go for a Boot Camp/CrossFit type program – one where you work out with the same group of people that will encourage you to keep going. Set goals too, like doing a 5k race and move on to Adventure Racing and/or extreme races like the Tough Mudder and Spartan races.