Gave this an initial whirl today. Hoped I might be able to turn them round on 3m30, just for ease of keeping track, and it turned out that this was just about the case. Although rest was closer to 10 seconds than 15.
Only did 12 today but that was because I had left it a bit late in the morning (8am) and the sun was getting strong. Could have gone on longer I think. Plan to try again another morning a bit earlier, with the aim of ticking off 20.
Mind kept wandering again.
planning a ride later, on the lit bike track after sundown.