Sprint distance - I read somewhere that a rider can to 20 revolutions all out. I used to go to courses and ride backwards from the line in a gear I thought I'd use in the sprint. If I had a lot of time, I'd ride backwards in 2 or 3 gears. Count 20 revs, look for a landmark. I've done most of the races for so long I no longer have to do that.
Then one year I watched a whole boatload of Tour sprint finishes just before a target race (to sprint like a pro, observe them, that's my motto). I realized most of them went much, much shorter in the sprints, like under 15 revs. So I readjusted my landmark to 15 revs (felt uncomfortable doing 8 revs - seemed too short) and although I didn't win, I did fine.
A notable exception is the Champs sprint in the Tour - they always go over 20 revs. It might be downhill or something, seems like the jump is not as important as the sprint itself.
Jump/Lead Out or follow? This is dependent on your jump.
If headwind, you ALWAYS follow until whenever you think is way too close, then you jump. In a slow uphill sprint (33 mph) I waited until 50 meters to jump and won by a decent margin (I posted some pics in the suffering pics thread). Guys just wither in the wind.
Tailwind, try to leadout if you don't have a decent jump. If you have a decent jump and you can go quite fast before using it, follow. The first jumper has the advantage because they have less of a chance of blowing up and the draft is not as significant, esp if the jump pulls you clear. In tailwind sprints I've led out from as far as 300-400 meters on very fast slight uphill sprints (Gimbles, 120 sprint, 42-46 mph top speed, go almost right after the exit ramp lanes after the bridge).
How to improve jump - pick better parents. From my experience, it's about 95% genetics. I never trained for sprinting but when I was 17 I couldn't bench 100 pounds (I weighed 103 to be fair) but when asked to do a test jump at a club ride/clinic, I could go from a standing start in one gear to 42 mph. I couldn't climb nor TT but I dreamed that I could. After that clinic (no one else broke 40 mph) I became a designated sprinter. It took some convincing that I could sprint but after I won a few field sprints, I was convinced.
Failing that, there are a lot of guys who sprint incorrectly and therefore don't use all their jump. Jump, in my definition, is sort of like a combination of peak and 5 second power. If you have an awesome jump, you can gap virtually anyone 10-15 feet in two downstrokes, maybe even just one, so it'll barely register in 1s peak. Within 3 pedal strokes you will be 20 feet clear and going away. It usually takes 50-150 meters for non-jump type sprinters to catch up to a jump type sprinter after a jump.
1. Learn to shift while under 100% load. I usually shift on the downstroke when I jump. In other words I slam the pedals down as I shift one gear higher. At Bethel, a sprint that normally starts flatter, goes up hill, then flattens out, I'll sprint in as many as 5 different gears if I lead out. You MUST have a good chain installation (it'll break otherwise and you'll face plant spectacularly) and perfectly adjusted gears to do this. My gears are adjusted to shift into smaller cogs perfectly. I have to tickle the shifter to get it to shift into larger cogs sometimes but that's my choice. I don't want the chain to hesitate when I want it to drop down to a smaller cog. Novice mistake when practicing this is to shift more than one gear at a time. You have to learn to exert 100% force on the bars and pedals and still have a delicate touch on the shifter.
2. You absolutely must have your bars and shifters positioned correctly. Forget the jacked look (levers up high, bars tilted to point to the moon). I watched guys crash because they can't even brake from the drops - stupid, stupid, stupid, and the guy broke a bazillion ribs and he's a lawyer to boot (so he's not dumb). You must be able to shift and brake without moving your hands from your sprinting position. Absolutely no question about that. If you have to wiggle your elbow to reach something when you're in the drops, move that something.
My rant on this bit:
Second one shows my bar/lever angle and how my medium sized hands with short fingers reaches everything.
3. Work on pulling up really, really hard. Lift - bent over rows, curls, butterflys, dead lifts anything and everything that helps you pull up on the bars when you're bent over. I don't work on my legs unless you could very infrequent 90 pound squats and 45 pound leg extensions as workouts. I regularly do pull downs, curls, bent over flys, dead lifts, and core stuff - situps and crunches. I also do work to protect my precious shoulder area - military press, bench, etc. I found my top speed improved with this type of work. My jump really didn't though.
I know when I'm sprinting hard because my stomach gets really sore. This hasn't happened to me yet and so I know I have more sprint in me. My max wattage at Bethel hasn't broken 1300 watts, but I was well over 1400 at the end of a 5+ hour long hot ride in California. I can't wait to unleash a "real" sprint.
4. Sprint. First have a good base. This means you can do a 2 or 3 hour harder ride without feeling new twinges. All the fatigue is familiar. Your body isnt' adapting to that ride, simply dealing with it. Then you work on sprints. Do lots and lots of sprints, with some kind of leadout. Failing a leadout, work on starting on a slight downhill so you're going 30 mph without killing yourself. Ideally you should find a 2 mile loop you can do over and over.
When you start getting a bit queasy and tired, do a few more. When you think you simply can't get going for another, do a few more. You'll be surprised at how resilient your body is. On days where I thought I'd do 2 or 3 sprints and then stop, I've gone to do 30 sprints or more (contesting half of them). I learned I could jump up to three times in one sprint - it takes me a couple months of sprinting to get that second jump, another couple for that third jump. But using a three jump sprint, I could beat much stronger Cat 2s and 1s by coaxing them to go earlier than they wanted to go, then out jumping them when everyone was in the wind already. I haven't gotten past a 1 jump sprint since 1995 but I hope to progress this year. It's like a video game. Gotta earn those jump bonus badges
5. Positioning/Awareness/Tactics - you seemed to have placed high very early on. This is good. But you should still focus on positioning both during the race and just before the sprint. Learn how to stay out of the wind. I posted a comment on a thread but felt it deserved some expansion so here it is:
I'm glad that you say you got blocked in rather than saying "then I moved right, a bunch of guys went down, and I got second". Awareness during a sprint is critical. Sometimes you get boxed in - in those instances, it's often because you focus on the wheels directly in front of you. If you can look up just a bit, to the racers in front of them, then you'll find it easier to read the field. For example, when driving around a corner (exit ramp, bend in road, etc, where you can see the road for a while), I look around the corner, not at the curb at the apex. Much better awareness of what's happening up front. I learned this the hard way while autocrossing - my times were horrible until I looked 2-3 turns down the course. Suddenly I was much smoother because I wasn't focused on the cone 15 feet in front of the car.
A good way to practice this is night riding. Point the light up and forward so you can't see the 10 feet in front of you. You learn to read the road up there, not under your tire, and you focus appropriately. This is especially true when mountain biking. A very zen like approach, you simply go as fast as you can, reacting to whatever you see 40 feet in front of you.
You mention you marked the guy but he took off early. If you're on a wheel of a guy going at 500 meters, it's usually a good bet to go with him. He'll blow at 200-250 (given normal efforts) or you can blow by him at 50 meters to go. If you're not on his wheel then you may not have a choice.
Even if you're strong, you can't bluff on your strength. At some point you're going to deal with guys just as strong as you are and your bluffing will fail. Then your tactical savvy will affect your results, and if you're a strong dumb rider, your results will reflect this. I'm pretty weak as a rider, no matter how much I train, but I ride as smart as I can and I manage to get some places and even win a race here and there. My sprint helps, of course, since that's what I use to place, but smart riding never hurts anyone.
Unfortunately, it comes down to genetics for the all out jump. But by working the sprint, tactics, and technique, you can do a lot to overcome the jump.