Scott, you are one of my heroes!
You are doing the right thing. Fitness is not just about times and personal records........it should be about doing what you love and enjoying the process too. Who cares if you walk the whole race? Who cares what your time is? Remember to be thankful that you are now in a body that CAN do these things.
Do it for those that can't. Do it because it is fun. Most importantly do it.
I'm very happy to hear that with all you've overcome (which, by the way, is A LOT) that you are making time to get your fitness in and you are losing the lbs you found. THAT is truly the picture of success.
Good to see ya back!