That was great - my back is now noticeably worse than it was before I started, but not actually painful, so we will consider that a win. (I would say it's about 60-70% better - will be careful not to compound it with reinjury or overuse.)
10 minutes warmup, then 6 x 3min hard,* 1min recovery, then cooldown jog home
43min total (c. 4 miles)
*I took out my Garmin for the first time this week in a while, and it doesn't seem to be working properly - I think it's probably just at the end of its lifespan. Have ordered a simpler Suunto monitor that I can use for HR zone training on runs and that is compatible with the system at the cycling studio. Hopefully that will suit my needs - I don't really need the GPS as such, but if this one turns out to be too minimal I may replace the Garmin next year.