How much time do you spend between sessions preparing, out of curiosity? Feel free to average over a campaign if you front-load a lot of the preparation.
As a general rule I try to spend two hours for each hour of play. That tends to achieve 'desirable' results. More can at times achieve better results, but really that's something adults can't expect to do consistantly. I think you can, if you are experienced enough, most of the time get by doing one hour of prep for each hour of play. However, in my experience this is generally risky and a lot of times I regret being so lazy.
In general, I've probably sit in at the tables of two dozen DMs. There is IMO a purely linear relationship between the quality of play and the effort the DM expends out of play. I'm sure that this isn't a hard rule and different DMs get different value out of their prep, and there are exceptions out there of DMs that are polymath phenoms that could appear on 'Whose Line is it Anyway' and be perfectly comfortable, but the average player who says, "I'm a great extemporaneous DM." is in my experience absolutely kidding himself.
I have in 30 years of play had zero enjoyable purely extemporaneous sessions. The is an inverse return on investment in the length of time something goes extemporaneous. An hour, or two, maybe four can work. A one off certainly. Sooner or later though you are in a work debt. The longer the campaign goes, the more it needs the blood and sweat put in much the same way that the bigger the building, the more you have to architect it and engineer it. There are maybe nine or ten agendas that a player can bring to the table. All but about three of them (fellowship, self-expression, fantasy) fall apart in the long run to extemporaneous play.
If I had to give one bit of advice to a new DM on how to improve his game, it would be prep more. Prep more, and that doesn't work then prep more efficiently. It's possible to prep the wrong stuff, but that's not usually the DMs problem. Do not set out with this fantasy that you are going to be this great improv DM. Chances are your table will fall as flat as improv plays, comedy, and movie scripts usually do in the hands of amateurs. Yes, RPGs are a different artistic/narrative medium, but the same general rule applies. You really try to improv and it can start great, but you'll soon end up as lost and muddled as the writer's of Lost or any other work where the writer made up crap but hadn't really done in world building. Your only saving grace will end up being in a work as emphermal as an RPG, the players will probably forget everything that happened a few sessions back anyway. A true genius can manage pulling things off the top of his head, but even a genius is probably going to produce better work improvising a head of time and recording his most interesting ideas. You want to have a great game, there is little subsitution for sweat. Don't as a starting writer go off imagining that your true stream of consciousness novel is going to beloved and widely regarded or will reflect what you are really capable of. If you want your improvisation to seem effortless and natural, practice it for hours ahead of time.
Anway, rant off. I'm passionate about this topic because I see the craft of GMing dying out there because no one wants to put in any work, and as a result the experience of play ends up suffering compared to other mediums - like CRPGs - where someone did put in work. It's the greatest threat to our hobby, and one that in all likelihood is going to all but kill it short term.