As a non-serious trainer, just someone wanting to get in shape, here is my experience.

I started in July and rode the bike path (about 10 miles) almost every day. Initially, it took me over 50 minutes to do this ride with the few traffic lights, slowing on the sharp corners and avoiding the people who like to use the whole width of the path. Within about 3 months my average time was down to 35-36 mins on the same trail and same obstacles. I never rode all out or anything but at a good clip and keeping my HR up above 140 (I'm 43).

What I take from that is that frequent, aerobic rides - even moderately short ones - really helps. I also lost 20 lbs doing that. I eventually got to the point that that bike path ride wasn't sufficient for a work out. It got too easy. I was doing that on my mtn bike so I bought a road bike and now do 25 mile rides in about 90 mins or so. My one minute HR recovery is 35-40 bpm and my resting HR is down to 55. I'm very happy with the progress I've made.

I think you should focus on 5-6 45min rides a week. Within a few months you will probably notice some big improvements. At that point, either just keep it up or increase your riding time or focus on a more specific type of training. When out of shape, you don't need anything fancy - just time in the seat and moderate effort.