I was able to get out on the Single Speed yesterday. I had take my computer off "Blue" (my trek 5200) and put it in my pocket to get time and HR functions. Since I couldn't see the computer, I had to wing it to judge my time and effort. The goal was to ride in zone 2 for 3 hrs. I ended with 2:30min mostly in zone 1 my avg HR was 121.
Although my avg HR was low, my legs got a good strength workout with riding on the Single Speed climbing some hills. I stayed seated and powered up the hills.
I stopped along the way to flipped the wheel around to change from freewheel to fixed. That was interesting. It was a little dangerous at first but once I learned to slow down using my legs instead of breaking I was fine. You have to plan your stops so you feet are in the right position to start up again. This time no crashes because I was constantly reminding myself that I was now in "fixed" mode.
Today I may do 1:30hrs and weights.