Rhythm: I'm no sprinter but have done a bit of work on indoor tracks in the winter under the instruction of men who know this game.
If you only have a few weeks do not expect a great increase in leg speed. The best result you will be able to obtain is in the healing stages of the kind of leg soreness sprint training will bring. This is not to discourage you. But to start sprinting or seeking a faster turnover has its price at the outset. Still, getting a jump before real indoor work starts is better than not.
Pick a stretch of road free of traffic because it's easy to get unglued and wobble a bit when first cranking very high revolutions. Plus a rider tends to look down at the feet and this is all it takes at 35 mph to get your front wheel cockeyed and you out into the main flow of traffic. Or into the bushes. (I've done this myself.)
Don't worry about a big gear. Go down on the small ring and spin whichever gear is easy. From a roll crank up to the fastest cadence you can and hold that for no more than 15 seconds. Do this several times with whatever rest you need between intervals. You are not training your lungs so no need to get right back at it. It's the legs' ability to move quickly you are working on. They move faster when fully rested. Later you can kill yourself with shorter rest times.
Experiment with gearings. Use the gear you can get to 140 rpm or better. This kind of speed will not come immediately. Or I should say it did not for me. Work on keeping the bike somewhat straight. Not to worry so much when sprinting your bike is wobbling a bit. Nobody sprints with a smooth bike attitude. Just stay on the road. This is always the best.
Simply try various sprint times starting at 15 seconds and lengthening them out to 2 minutes. At 2 minutes you are apt to slow but it's still a good goal time. The whole point of this kind of thing is to tear leg muscle on a microscopic level. This is how you build specific muscle that is tailored for cycling. Weight lifting with the legs helps but does not teach the leg tissue to fire in just the right sequence. Specific training at its best.
Do this kind of training a little at first (10x15 seconds first day) and slowly increase as each day goes by. Do not do a whole bunch at first unless having a mate help you get a leg over is what you're after. This kind of thing can make you very sore.
Ride your usual distance and add the sprints sometime in the middle. Use the last half to warm down and disperse lactic acid.
All coaches have their own patterns of training. Mainly they vary by target cadence times and work times. But if you simply get out and practice spinning faster by the time you get indoors you'll be a lot better off. A coach is very handy too.
Hope this is of some help.
