Let's say you have some pace at which you normally ride. You get fitter and stronger by riding both much harder than that pace and much easier than that pace (to recover). You don't get much faster by riding at your normal pace, though that does give you more endurance. So you need all three things: Hard, normal, easy. It's going much harder than normal that makes you faster, but then you need to recover from those efforts.

Like umd says, ignore the trainer speed, except that I watch my trainer speed at a particular heart rate to see if I'm getting faster over time, and during a workout to see if I'm getting cooked and slowing down while maintaining the target HR. But don't compare trainer speed to road speed.

Almost everyone has more trouble getting their HR up on a trainer compared with on the road. In fact, I think my HR zones shift down on the trainer a bit. It's because the trainer doesn't excite you and the road does. Excitement gets your HR up. I can vary my HR at the same trainer speed by 10 beats, just depending on what I think about.

So you did one 30 minute LT or sub-threshold interval. You should do at least two of those, with 10-15 minutes recovery between them. That helps your body learn to recover while still pedaling. But 30 minutes is a little long. I rather like 3-4 15 minute intervals instead of one or two long ones. They seem more doable on the trainer. Others like two 20 minute intervals. You'll find that your HR will come up a lot faster in the second and successive intervals, maybe only 3 minutes instead of 15.

I think you get the 30 minutes. That's the reason people train with power now instead of HR, so they can more accurately judge the quantity of their effort. But a power meter isn't cheap, so it has to be pretty important to you.

You can also do shorter even harder intervals. 3-4 minutes is a nice, painful period of time. Do about 4 of those.