I used incline during the efforts - starting at 1.5% and then lowering to 1.0% when that felt tough. I perhaps didn't push the first one enough and I was only at HR 152 by the end of the first 10 minutes (covering 3.24kms or 3:05/km pace).
Second one was much better - started quicker and hot the zone no problem. Clocked up 3.3kms (3:02/km pace). Max HR 154.
Last one much the same as the previous one except the heat rate went a little higher. Again 3.3kms at 3:02/km pace. Max HR 156.
Total 9.84kms in the 30 minutes.
I was getting a massive 5 minutes recovery between efforts which helps. Felt good today - could have pumped out another one I'd say.
A 1km warm-down and that gives me 12.8kms for the day. Really enjoyed that session.