Bike fit / correct riding position

Last summer (maybe the summer before that?) I was playing around with TT bars, I did a test in the park (on what was meant to be a rest day) that saw me holding 20 mph with my heart rate at or below 120 bpm for an hour.

Now I'm back to wanting clip ons.

Did a 30 kph ride at 130bpm a couple days ago. The main aim being to keep heartrate low and steady, not to go fast.

hmmm.