You DO have to be a certified jockey, you can't just walk in and hop on...these guys (and girls sometimes) are very experienced
Not true, at least not in my experience. I worked as an exercise rider on a farm, and we worked at racing speed up to 5/8s. We would also occassionaly haul horses to the local track and gallop/work at the track. No license was required. We did have to be working for a licensed trainer, however. You can't haul your pleasure horse to the track and take a spin, that's true. Perhaps that's different is different states, but I've never heard of exercise riders needing to be licensed. To actually ride in a race, yes. A trainer would have to put you up for your apprentice's license, and you have to gallop and break from the gate in front of the stewards to be awarded an apprentice's license.
Starting out riding by riding at the track would be *very* tough. The horses are all fit; and you have to deal with all the traffic on the track in the mornings. Much easier to start on the farm breaking, and get fit as the horses get fit, before attempting to ride at the track. The only way to break into riding cold, at the track, is to start out grooming and hot walking and progress to riding the pony horse during morning work. Would still be very tough. The pony just doesn't prepare you for the racehorses. You have to be incredibly fit to hold a fit racehorse; holding a fit racehorse with a lot of other traffic on the track is incredibley tough.
To answer your other questions: it was incredibly fun. Working at racing speed ("working" as opposed to "galloping") is a huge rush; working from the gate is an even bigger rush.
Back in the late 80s I got $5 - $6 dollars a head, late 90s I got $7 a head, up to $10 for really tough horses the other riders wouldn't get on. I didn't do any grooming; just tacked up the horse, galloped, hosed it off and put in on the hot walker, and cleaned my tack at the end of the day. You can ride 4 an hour if you're hustling. If you're working with a good groom who does the tacking and cooling down, and they bring a tacked horse out to you and you can hop from one to the other, you can do 6 an hour. No idea what the going rate is now.