You're on the right track. At first, do whatever you can comfortably. Then start building on time. If you can only do 30 minutes to start, then start with that & gradually work up to an hour per work-out. Also toss in a brief warming up & cooling off period of time at slower speeds.
Don't worry about how far you've gone or how fast you've gone. That will come with time. What you should concentrate on is that magic word called 'cadence'. Cadence is the rpm of the pedals. Try to maintain 70 - 90 rpm by up-shifting or downshifting into the right gear using moderate pedal pressure. If pedaling becomes difficult, downshift. Too easy, up-shift.
I'll put one or two links below.