Mish said:
The reason the pros push a seeimingly large gear at high cadences is because they are riding a lot faster than you. If you are sitting in the draft in the middle of a peloton that is going 55 km/h it's easy to ride on the 15 and make 90 rpm. The power output would probably be similar to riding at 38 km/h on the 18.
90 rpm is considered the most efficient cadence, and many top riders actually exceed that. I myself ride at 100+, but then I can because Holland and Belgium are fairly flat.
If you train constantly on a big gear you will only get good at riding slowly in a big gear (you would need to be pro tour standard to ride on the 15 all day at an efficient cadence) and you will ruin your knees. If you ever start racing you will also find that riding a big gear means you cannot respond quickly to the constant accelorations in the pack, meaning you will have to close gaps, get more wind and you will get dropped very quickly.
It's actually totally possible to ride top level races at high speed on the 17 - 19. I ride Elite crits on the 17, clicking up to 19 at times out of sharp corners, with cadence around 100. I
sprint on the 15 dude!
My advice is to do much of your training at a higher rpm, and save the high power stuff for your intervals. Even then, keep the revs high - works your cardio better, less stress on the legs - raises V02 max, whilst allowing for faster recovery, and also trains you for the accelorations you will need whilst racing.
If you want to know how a pro trains you'd be surprised. Lots and lots of hours riding an easy gear (on the inner ring) at high cadence, sprints (often also on the inner ring) and racing to build form and clock up "intensity" miles. None of them are grinding along in 53 x 14, except when the speed is right up in a race, or if they are time trialling - and then those guys are strong enough to push those gears at higher cadence for a decent duration.
I strongly advise against continuing with your current training.