zoobrana,
Let me be the first to welcome you to the Forum, and congratulate you on what most of us here would consider an excellent career choice. To me the hardest on the feet and legs is to stand in one position or area on concrete floors. BTW do your feet a favor and with your first paycheck get a pair of comfortable (Wolverines are my choice, not cheap, but not overly pricey either) work boots. Depending on the job, you never know when your going to be called upon to "python" yourself into some awful contortionist nightmare to ply your trade. Boiler tube welders do this daily.
Common sense dictates that if you take care of your body and stay in shape, it will serve you well, whether your an office puke (like me now) or climbing the iron. You don't have to be an Olympian, and I think it is more beneficial to limber/flexible. I've done this kinda work for "a few years" and I gots more aches and pains from broken bones bouncing off of rocks having fun on skis, sno-machines, bikes, rafts, etc (or maybe those are just masking the true injuries from burnin rod....NOT!)
Main thing is to take care of your eyes, lungs and back! Use all PPE provided and properly, and follow safe lifting proceedures and body positioning. Even when I was young, I'd often get ridiculed for using cranes and come-a-longs instead of macho-man-ing it up there. When asked if I had a bad back..I reply "No I got a good back and it's gonna stay that way!!!"
Gotta go now and put some linament on my sore shoulder and elbow from whipping the rod too much the other day. That's an 8-1/2' Fenwick Fly Rod BTW!

Enjoy and take care

I second Superflux's suggestion to buy good boots.  For me it's Redwings, find a boot that is comfortable and stay with it.  Don't be afraid to spend a couple hundred for a decent boot that will last.  Better to spend $200 for boots that will last 2-3 yrs than spend $50 a pair that need to be replaced every 3-6 months.  Also get waterproof ones if you'll be working outside, 10hrs+ with wet feet is no fun.

Get the best you can in any protective equipment for your job(and USE it!  Especially EAR PLUGS and safety glasses!).  The extra expense now will pay off 20-30 yrs in better health.

I spent 16 yrs repairing heavy equipment for a dirt & pipe contractor.  Lots of welding in tough positions(laying in the mud under D8's welding cracked track frames, crawling inside scrapers welding tailgates, hanging on a ladder welding cracked excavator booms, laying in a trench 12 ft below a riverbed welding casing pipe for watermain) so I can say, yes there are some areas that can be a bit tough on the body but if you use your head you can minimize the damage.  Biggest thing is don't get in a hurry and do stupid things just to get done quick.  Take your time and work smart, take that extra minute to put earplugs in before you pick up a hammer and beat that piece of steel into submission.  Walk over to the truck and get your safety glasses before you pick up the grinder and grind that tiny burr off("I only needed the grinder for a second.  Now I can't see out of that eye." - it does happen).

Take care of your body now while you're young you'll be glad you did later on.  If you wind up working for someone who doesn't think your health and safety is more important than their bottom line, pack up your tools and move on.  If you are good at what you do and work hard, there is always another job waiting.

Jim S

Are you just going to be a service fitter or are you going to see the industrial side of our trade as well?  If you get to see the industrail side, you'll get ya a taste of a body beetin sometimes brotha.;-)  You'll feel it if you get out in the field I promise.  Don't get me wrong, it's not all bad all of the time, but there will be some instances where you will be wishing you were in a closet or under a house.  Or are you just going to be a service fitter working on HVAC stuff all the time?  If you get a chance to get out on some shutdowns or industrial construction, jump on it and get out there and get ya some.  I don't know how well I'd do under a house with spiders and other types of creepy crawlies so I'll give you service guys some props there.

Take care of your lungs. One of my Welding Engineering professors died a slow and painful death from lung cancer - in his late 50's.  It is strange because he never smoked a cigarette in his entire life.  One of the things I remember about the welding lab was poor ventilation.  It is a controversial topic no doubt.  He was very hands on and did lots of welding development for his consulting job with oil industry companies.  He was also there for years watching students weld.  It might have been something else that caused it, but it makes me suspicious.  The entire flap lately is to avoid chrome vapor like the plague.  If you are a welder, use good ventilation or wear a respirator for toxic stuff.