You're not that old to start riding ;) My dad is almost 50 and he's just picked it up this year and is doing really well!! It's never too late to start :)
As this is only your second lesson, of course your instructor will be putting you on a lazy horse. If you were put on a sensitive horse that did everything it was told (even if you don't mean to tell it something!) you'd probably end up on the ground because your balance isn't yet sufficient to tackle something more lively. Don't blame the horse at this stage, your aids will be very unclear and so the horse will be slightly evading them at the moment- which is why you want a lazy, dead quiet horse to learn on!!
As for where you should be in your first lesson, well, it depends on you really. Some people are naturals at it and will be cantering and balanced on their second lesson. Others/most will still be very shakey and not understanding of exactly what to do at this stage.
All you should be doing is really getting a feel for the movement of the horse and learning to go with that movement. Learning to rise/post trot is actually extremely difficult!! You forget how hard it is when you've been doing it 'forever'. Teaching my dad to ride really opened my eyes up to just how hard it is to learn. Initially you'll only be able to ride a few strides then lose your rythem, but as you have more lessons, you will start to get the feel of the movement of the trot and your muscles and balance will be more in tune with riding. Soon enough rising will be the most natural thing in the world and you'll hate sitting haha!
Watching videos and reading is all good and well, but there is no way you can just jump on a horse, pick up the reins and be able to get collection straight away. So many people start out thinking they're experts because they've read books, but then they get on a hrose and realise it's just so different. Every horse is different, so you have to adapt to riding differently. Once you get mroe experience on the one horse, you should try to get a chance to ride as many as you can, so that you learn to adapt to the differences and understand that you can't ride in exactly the same way on every horse.
Exercises, well, at this stage there's not a heap you can do other than get in as many riding hours as you can. The fitter you are the easier you will find it to pick riding up as you won't get so fatigued. I try to run a few times a week and also go to the gym. Most importantly you want to have a nice toned belly. I use my stomach muscles more than any other muscle in my body when I ride, I use them to slow the horse, to turn, to half halt, to get collection, etc etc. So tone that tummy! Sit ups are great ;)
Also, strengthen your legs by sitting on a chair (when you're watching TV ;)) and lifting one leg parallel to the floor, and hold it for as long as you can, then do the other leg.
Stretching is good, you want to be nice and flexible. Pilates is great, you need to learn how to seperate your body so that you can use your hands independantly of your seat, legs etc. Even now, after riding for almost 17 years, many horse including breakers and ottb's, I try to have lunge lessons as frequently as possible with no reins and stirrups in order to make sure I am still using my aids independantly.
To see if you are good at using your tummy to stop the horse and balance yourself, sit on the edge of a plastic chair, both feet flat on the floor and with your pelvic/6 pack muscles only, try to tip the chair forward while keeping your body straight. Its harder than it sounds if you do it right!!