I wasn't in long, bad ankle, but I did go through basic so I can tell you what it was like there. When I was there in late 1989 and early 1990 there were several guys in their late 20s and early 30s (I'm not certain anymore if the oldest guy was 34, 35 or 36, but I think he was 36). The older guys, especially those in their 30s, were very well respected. Sure, they couldn't always keep up physically, but we recognized they had a lot to offer, and we certainly respected what they were trying to do. The drill sergeants certainly gave them hell, often using their age as the "weapon" to use against them, but they gave everyone hell and tried to find something to use against them. Even the drill sergeants seemed to have a lot of respect for them and probably did treat them a tad better than the rest of us. The hardest part is there is a lot of stupid BS in basic, and at 19 I recognized that. At 34 it will be glaring, and having some 22-27 year old kids in your face and treating you like you were a total idiot for a couple months can get frustrating (the physical gets easier as it gets harder because you will get in much better shape quickly, the stupidity is always there, but usually somewhat entertaining at least). On the age of drill sergeants, most of my drill sergeants were older and more mature, many if not most being Vietnam vets, but many were in their mid-20s as well.
Now if you are out of shape I would advise against infantry especially, or any of the combat arms really. They are very physical, and unrelenting (many military jobs are nearly 9-5 type full-time jobs, the combat arms are no where near that and are really a way of life). If you want "action", EOD (explosive ordinance disposal) or MP (military police) would probably fit (there is action, but they aren't quite as physically demanding), and they would also probably appreciate the extra maturity that comes with age (both are pretty mentally demanding and require mature decision making). There are also plenty of desk jobs that contribute (if I would be to go back I'd probably go in as a mental health counselor- lots of PTSD and adjustment problems going on with many of the troops returning from Iraq). There are quite a few jobs to choose from:
http://goarmy.com/JobCatList.do
As for some of the restrictions due to your age:
-No ROTC: The age limit to join Army ROTC is 26 and of course you have to be in a college program
-You are at the age limit to enlist in the Active Army. The age limit is 34, if you go reserves it is 39 (not sure about the National Guard)
-OCS (Officer Candidate School): I'm not sure on this one. According to the Army's recruitment website (goarmy.com) the limit is still 29 (you must get your commission by your 30th birthday). However I've been reading that the Army has, or is about to, raise the age to 42. You may want to ask a recruiter about this. You will need a BA or BS degree first however (Reserves and/or National Guard commissions are possible I believe at 60 credits if you are in a degree program for your BS/BA and will finish in a certain time frame).
-OCS at the state level: Many states' National Guards have state level OCS in addition to officers training in the Army's OCS. Those states that have their own OCS (most do) sometimes have different age limits so you may be able to go through OCS this way.
-You may wish to wait until you are done with school. If you plan on grad school for something like psychology, if you plan to become clergy, or if you plan on medical or law school, there are different commissioning programs for these kind of jobs and they have different age requirements. At least one of the commissioning routes for psychology allows a max. age of 43 and I'm not even sure if there is an age limit for chaplains anymore (I can't find a maximum age online anywhere), last time I saw one (years ago) it was 40-something.
-Keep in mind that the Army has the most liberal age restrictions, so going to the Air Force, Navy, Coast Guard, or Marines if you are too old for what you want won't work (they generally have lower age restrictions).
-Most age restrictions are waivable, so if your recruiter says you are too old for the program you want, ask for him/her to pursue a waiver. If your recruiter won't go through the work, go to another recruiting office and get another opinion before you give up.
If you enlist with an Associates degree you will go in as an E-3 (Private First Class) instead of as an E-1 (Private). With 30 credits you go in as E-2 (Private), at 60 you go in as an E-2 (Private), at 60 you go in as an E-3, and with a BA/BS degree you can enlist as an E-4 (Specialist). In each case this gives you a little more money, a tad more respect, and a slight leg up on someone coming in as an E-1 (it can take 1 1/2-5 years to make E-4, typically 2-3).
You will probably want to quit smoking now. Your lung power will increase, and tobacco products are banned during Basic Training anyway (trust me, Basic Training is not a good time for niccotine withdrawal).
Losing weight is a good idea. At least, do some working out so you are used to doing the pushups, situps and running. You don't need to be in perfect condition, basic will shape you up, and if you are in too bad shape there is the fitness training company (FTC) if they need to put you there (I assume they still have them, though the FTC is not someplace you want to be).
Look into things carefully, but I would strongly encourage going in if you think you might want to. At 34 you are old enough to realize that a couple years really isn't a long time. Take a 2 or 3 year enlistment if you really aren't sure if you'd like it. Even 4 years goes by pretty quickly. Then, there is always the Reserves if the money (it doesn't pay well) or the time committment keeps you away from the Active Army, just keep in mind that you can/will be called up and the enlistments tend to be longer, 3-6 years and most are 6 years. The Reserves/National Guard drill once a month (2 1/2 days, usually Friday night through Sunday evening) then 2 weeks usually in the summer when not called to active duty so most of the time it isn't quite as much of a committment (you can still have your normal life most of the time). With the Reserves/Guard these days you may spend more active duty time than someone on a 2 or even 3 year Active Duty contract, but the time will be more spread out so you will be at home more, and your unit will be based at home when not deployed (i.e. unlike Active Duty, you won't be constantly moving).
Miscellaneous links:
www.goarmy.com (recruiting)
http://www.usarec.army.mil/hq/apa/rc/apft.htm (Army fitness standards)
http://www.usarec.army.mil/hq/apa/rc/weight.htm (Weight tables- be sure you aren't over the maximum weight limit)
http://www.dod.mil/dfas/money/milpay/pay/paytable2005-rev1.pdf (PDF chart with military base pay rates, housing allowance, food allowance, reserve pay, family separation allowances, and other pay catagories)