I don't have easy access to a treadmill.
As Jeremy Howard mentioned in his talk, SRSing (is that a word?) is exhausting. Like him, after a period of about 20 minutes, I often reach a level of fatigue that makes it difficult to continue studying.
I 1st read about the "treadmill method" on Seth Robert's blog & found it highly effective. Like Mr. Howard, I could study for hours without becoming bored; unlike Mr. Howard I didn't track my error rate, so unfortunately I can't offer more evidence for that effect. As I mentioned, the only problem here is that I don't have easy access to a treadmill. My gym is quite far & it is impractical to go there every day, while I desire to SRS every day.
Fatefully, I stumbled across this study:
Brief Wakeful Resting Boosts New Memories Over the Long Term.
The gist of it is that subjects who spent 10 minutes relaxing after learning had improved long term memory vs. subjects who went on to perform some other cognitively demanding task.
Since reading the study, I have altered my SRS review method. I now study for about 20 or 30 minutes in a stretch, then spend 10 minutes lying on my back practicing "wakeful resting". Unfortunately, I only recently discovered & became interested in QS, so I haven't been recording data before & after. All I have so far are my subjective judgments.
My feeling is that this method is as or more useful than walking on a treadmill. For me, it is much more convenient. It has the added benefit of maybe enhancing long term recall, which is my end-goal with SRSing. It has the same effect of extending one's ability to study for long stretches of time. It also leaves one feeling refreshed & ready for the next task. In fact, I often find that after 10 minutes of wakeful resting, I'm literally jumping to my feet when my phone's alarm goes off.
I would love it if someone with a good data collection with SRS could adopt this method & track changes in error rates & so forth, as well as all of the quantitative elements I've mentioned.
If not, at least I've shared an effective method that will hopefully be useful to someone here.