To increase fitness the bulk of your work should be at an aerobic heart rate, and throw in the intervals sessions no more than once or twice a week, assuming 5-6 sessions a week in total. If you were training for an event the advice would be different.
On those kind of bikes HR is a much more realistic measure of effort than speed for a given duration. On my turbo trainer (road bike on a frame) at home, I work either in power (Watts) or HR.
Don't worry about standing, only useful for hills or sprinting. And when you look for cycling shorts, get women's specific, padding in a different place.