I need some good advice about using my fluid trainer

I am have done about 125 miles a week for the last 6 months, on some hilly terrain. Now I am using my indoor trainer for 1/2 to 3/4 hours per day, in the morning.

I dont want to lose the fitness that I have now. Unfortunatey I dont have a heart monitor. Is it better to ride at a high cadence than a lower one with a higher gear? Is 30 to 45 minutes a day enough?

My workout is like this

Ten minute warm up, cadence 60- high gear. Now the rest of the workout is mainly a cadence of 80 and during this time, I am breathing/sweating heavily and my legs are buring a bit, but only very slightly.

This is my main question: About two or three times during the workout I shift into a higher gear and push myself to a cadence of 90 to 100. I can't maintain this very long (only 30 to 45 seconds). I can feel my heart pounding, I have to gulp for air and my legs burn heavily - I push myself until the pain makes me have to stop. Does this qualify as an 'interval'? Will it keep my legs strong?

When I start to ride again in the spring, I would like to not have to struggle in order to get to where I am now.

Thanks