Ulrich, best practice is to wear maille every day, even if its only for an hour or two. Don't just sit or stand around, move- do deep knee bends, jog, run, hike- your muscles will get acclimated to the stress if you do it more regularly. A few times a year isn't enough to build the required tolerance to bear the weight- as such you'll always feel tired and drained even if all you did was stand around wearing it.
Posture is key, Nathan pointed that out- if you're used to slouching you'll need to train yourself to stand upright, shoulders back. It does help if there are multiple areas were the maille is drawn in close to the body- a belt at the waist, ties at the elbow and even the wrist can go a long way toward mitigating the stress factor on your back.
Or, you can opt for aluminum maille and bypass the whole weight issue- I just got ahold of a full hauberk that might weigh 10 lbs- light enough so I don't even notice that it is on. The trade off is that it is lighter so it is not protective, but if you're not fighting in it you needn't worry about wearing heavy steel unless you're so inclined, as it seems you are. If that is the case- wear it every day and get to the point where you can put it on yourself. There may always be something you'll need assistance with, especially with plate (although I watched Nathan don and point his plate arms and legs whilst talking with me and Sir Edward at VARF last year) but the more you can do yourself, the better off you'll be. Once you get the hang of it, donning and taking off maille is no big deal...I do it all the time.
With regard to your slight build...it may surprise you to know that I was once as slim as you...in fact, I hit a growth spurt late in my teen years so I shot up from 5'2" my junior year to 5'9" my senior year and hit 6' by the time I turned 18- but I still only weighed 128 lbs! I started putting on weight in my mid-20s and hovered at or near 170lbs for the next ten or so years, and I worked out regularly. Now that I'm nearing 40, I'm finding keeping weight OFF to be the bear! I miss the days of high metabolism and eat-anything diet! I'm also pretty lazy when it comes to the gym these days...but muscle memory being what it is, it doesn't take me too long to get back in the swing of things...altho it does take longer than it used to.