Sounds good. At this stage pretty much anything you do will get you in better shape as long as you spend enough time on the bike. Riding at different intensities (i.e. intervals, stuctured or not), will cause more improvement and will make riding more interesting than always riding the same speed.

if you're calculating HRmax from a formula there is a good chance it is wrong for you. Better to do a field test, either for HRmax or for LTHR, which is a bit more useful and not so unpleasant a test.