Congratulations on being under 300! Does the idea "my body, my science experiment" appeal to you? There is lots you can tinker with to figure out what works best for you.
Please read the book Slow Fat Triathlete, by Jayne Williams. I read it in 2004 when I was 48 years old. I had been diagnosed with diabetes 6 months earlier weighing about 215 (at 5'6") and had gone low carb and the weight was beginning to come off. Less than a year later I did my first sprint triathlon, and two years after that I did a half ironman http://deeplanguage.blogspot.com/200...lina-half.html. I had never been a runner or a member of a swim team. She argues we can live our athletic dreams in the bodies we have, and (what was most crucial to me), that you don't have to be good at something to enjoy doing it. I think it would be a helpful book even if your athletic dream is something very different from triathlons.
Here are a couple of examples of triathletes racing in the Athena category: http://www.saroy.net/2007/05/combat-...on-race-r.html and http://www.runhollyrun.com/?p=465. I think you are looking for something more like adventure racing or even Muddy buddy.