I tried doing 30 seconds of force + 30 seconds easy; three such 30/30's made one set. I rode easily for 5 minutes between sets, and did four sets. After that, I worked to stay around 15 mph as much as I could on the way home.
When I checked my ride data, I was surprised that I had spent only 25 seconds in my max heart range. Out of a total of six minutes of pushing very hard!
Or was I working less hard than I believed I was? Here is where a power meter would be helpful. I came close to taking the plunge and buying one, when up came the need for a root canal. Dental work is frightfully expensive, so the power meter has to wait.
Until I can get one, I'll have to keep on doing what I've been doing. And there are railroad tracks so close to home, I might be able to get in a bit of work on the railroad, after all!