Originally Posted by 2manybikes
Just go by time and effort. It does not matter how fast or how far, only how hard and how long.
If you like your tri bike, keep it, it's not worth much as a used bike, and new ones are a lot more. You'll never replace it for anywhere near what you can get for it.
+1
It's all about the time, and I'd recommend dialing back the pace so that you can ride farther comfortably. You want to stress your body, but you don't want to kill it. If you can fit it into your schedule, a slower 2 hour ride will confer more benefits than a faster 1 hour ride.
And 16 minutes for 3.6 miles isn't bad.
As for your bike, you can get more robust wheels for it, and keep the current ones for when you're lighter.